Class 8 Exam  >  Class 8 Notes  >  Mathematics (Maths) Class 8  >  Mensuration - II (Volumes and Surface Areas of a Cuboid and a Cube) - EXERCISE 21.4

Mensuration - II (Volumes and Surface Areas of a Cuboid and a Cube) - EXERCISE 21.4 | Mathematics (Maths) Class 8 PDF Download

Download, print and study this document offline
Please wait while the PDF view is loading
 Page 1


 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
EXERCISE 21.4                                                 PAGE NO: 21.30 
 
1. Find the length of the longest rod that can be placed in a room 12 m long, 9 m 
broad and 8 m high. 
Solution: 
Given details are, 
Length of room = 12 m 
Breadth of room = 9m 
Height of room = 8m 
So, 
Length of longest rod that can be placed in room = diagonal of room (cuboid)  
                                                                               = v(l
2
 + b
2
 + h
2
) 
                                                                               = v(12
2
 + 9
2
 + 8
2
) 
                                                                               = v(144+81+64) 
                                                                               = v(289) 
                                                                               = 17m 
 
2. If V is the volume of a cuboid of dimensions a, b, c and S is its surface area, then 
prove that 1/V = 2/S (1/a + 1/b + 1/c) 
Solution: 
Let us consider,  
V = volume of cuboid 
S = surface area of cuboid 
Dimensions of cuboid = a, b, c 
So, 
S = 2 (ab + bc + ca)  
V = abc 
S/V = 2 (ab + bc + ca) / abc 
       = 2[(ab/abc) + (bc/abc) + (ca/abc)] 
       = 2 (1/a + 1/b + 1/c) 
1/V = 2/S (1/a + 1/b + 1/c) 
Hence proved. 
 
3. The areas of three adjacent faces of a cuboid are x, y, and z. If the volume is V, 
prove that V
2
 = xyz. 
Solution: 
Let us consider,  
Areas of three faces of cuboid as x,y,z  
So, Let length of cuboid be = l 
Page 2


 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
EXERCISE 21.4                                                 PAGE NO: 21.30 
 
1. Find the length of the longest rod that can be placed in a room 12 m long, 9 m 
broad and 8 m high. 
Solution: 
Given details are, 
Length of room = 12 m 
Breadth of room = 9m 
Height of room = 8m 
So, 
Length of longest rod that can be placed in room = diagonal of room (cuboid)  
                                                                               = v(l
2
 + b
2
 + h
2
) 
                                                                               = v(12
2
 + 9
2
 + 8
2
) 
                                                                               = v(144+81+64) 
                                                                               = v(289) 
                                                                               = 17m 
 
2. If V is the volume of a cuboid of dimensions a, b, c and S is its surface area, then 
prove that 1/V = 2/S (1/a + 1/b + 1/c) 
Solution: 
Let us consider,  
V = volume of cuboid 
S = surface area of cuboid 
Dimensions of cuboid = a, b, c 
So, 
S = 2 (ab + bc + ca)  
V = abc 
S/V = 2 (ab + bc + ca) / abc 
       = 2[(ab/abc) + (bc/abc) + (ca/abc)] 
       = 2 (1/a + 1/b + 1/c) 
1/V = 2/S (1/a + 1/b + 1/c) 
Hence proved. 
 
3. The areas of three adjacent faces of a cuboid are x, y, and z. If the volume is V, 
prove that V
2
 = xyz. 
Solution: 
Let us consider,  
Areas of three faces of cuboid as x,y,z  
So, Let length of cuboid be = l 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth of cuboid be = b 
Height of cuboid be = h 
Let, x = l×b 
y = b×h 
z = h×l 
Else we can write as 
xyz = l
2
 b
2
 h
2
….. (i) 
 
If ‘V’ is volume of cuboid = V = lbh  
V
2
 = l
2
 b
2
 h
2
 = xyz …… from (i) 
? V
2
 = xyz 
Hence proved. 
 
4. A rectangular water reservoir contains 105 m
3
 of water. Find the depth of the 
water in the reservoir if its base measures 12 m by 3.5 m. 
Solution: 
Given details are, 
Capacity of water reservoir = 105 m
3
 
Length of base of reservoir = 12 m 
Width of base = 3.5 m 
Let the depth of reservoir be ‘h’ m 
l × b × h = 105 
h = 105 / (l × b) 
   = 105 / (12×3.5) 
   = 105/42 
   = 2.5m 
? Depth of reservoir is 2.5 m 
 
5. Cubes A, B, C having edges 18 cm, 24 cm and 30 cm respectively are melted and 
moulded into a new cube D. Find the edge of the bigger cube D. 
Solution: 
Given details are, 
Edge length of cube A = 18 cm 
Edge length of cube B = 24 cm 
Edge length of cube C = 30 cm 
Then, 
Volume of cube A = v
1
 = 18
3
 = 5832cm
3
 
Volume of cube B = v
2
 = 24
3
 = 13824cm
3
  
Volume of cube C = v
3
 = 30
3
 = 27000cm
3
 
Page 3


 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
EXERCISE 21.4                                                 PAGE NO: 21.30 
 
1. Find the length of the longest rod that can be placed in a room 12 m long, 9 m 
broad and 8 m high. 
Solution: 
Given details are, 
Length of room = 12 m 
Breadth of room = 9m 
Height of room = 8m 
So, 
Length of longest rod that can be placed in room = diagonal of room (cuboid)  
                                                                               = v(l
2
 + b
2
 + h
2
) 
                                                                               = v(12
2
 + 9
2
 + 8
2
) 
                                                                               = v(144+81+64) 
                                                                               = v(289) 
                                                                               = 17m 
 
2. If V is the volume of a cuboid of dimensions a, b, c and S is its surface area, then 
prove that 1/V = 2/S (1/a + 1/b + 1/c) 
Solution: 
Let us consider,  
V = volume of cuboid 
S = surface area of cuboid 
Dimensions of cuboid = a, b, c 
So, 
S = 2 (ab + bc + ca)  
V = abc 
S/V = 2 (ab + bc + ca) / abc 
       = 2[(ab/abc) + (bc/abc) + (ca/abc)] 
       = 2 (1/a + 1/b + 1/c) 
1/V = 2/S (1/a + 1/b + 1/c) 
Hence proved. 
 
3. The areas of three adjacent faces of a cuboid are x, y, and z. If the volume is V, 
prove that V
2
 = xyz. 
Solution: 
Let us consider,  
Areas of three faces of cuboid as x,y,z  
So, Let length of cuboid be = l 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth of cuboid be = b 
Height of cuboid be = h 
Let, x = l×b 
y = b×h 
z = h×l 
Else we can write as 
xyz = l
2
 b
2
 h
2
….. (i) 
 
If ‘V’ is volume of cuboid = V = lbh  
V
2
 = l
2
 b
2
 h
2
 = xyz …… from (i) 
? V
2
 = xyz 
Hence proved. 
 
4. A rectangular water reservoir contains 105 m
3
 of water. Find the depth of the 
water in the reservoir if its base measures 12 m by 3.5 m. 
Solution: 
Given details are, 
Capacity of water reservoir = 105 m
3
 
Length of base of reservoir = 12 m 
Width of base = 3.5 m 
Let the depth of reservoir be ‘h’ m 
l × b × h = 105 
h = 105 / (l × b) 
   = 105 / (12×3.5) 
   = 105/42 
   = 2.5m 
? Depth of reservoir is 2.5 m 
 
5. Cubes A, B, C having edges 18 cm, 24 cm and 30 cm respectively are melted and 
moulded into a new cube D. Find the edge of the bigger cube D. 
Solution: 
Given details are, 
Edge length of cube A = 18 cm 
Edge length of cube B = 24 cm 
Edge length of cube C = 30 cm 
Then, 
Volume of cube A = v
1
 = 18
3
 = 5832cm
3
 
Volume of cube B = v
2
 = 24
3
 = 13824cm
3
  
Volume of cube C = v
3
 = 30
3
 = 27000cm
3
 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Total volume of cube A,B,C = 5832 + 13824 + 27000 = 46656 cm
3
 
 
Let ‘a’ be the length of edge of newly formed cube. 
a
3
 = 46656 
a =
 
3
v(46656) 
   = 36 
? Edge of bigger cube is 36cm 
 
6. The breadth of a room is twice its height, one half of its length and the volume of 
the room is 512 cu. Dm. Find its dimensions. 
Solution: 
Given, 
Breadth of room is twice of its height, b = 2h or h = b/2 … (i) 
Breadth is one half of length, b = l/2 or l = 2b … (ii)  
Volume of the room = lbh = 512 dm
3
 … (iii) 
By substituting (i) and (ii) in (iii) 
2b × b × b/2 = 512 
b
3
 = 512 
b = 
3
v(512) 
   = 8 
? Breadth of cube = b = 8 dm 
Length of cube = 2b = 2×8 = 16 dm 
Height of cube = b/2 = 8/2 = 4 dm  
 
7. A closed iron tank 12 m long, 9 m wide and 4 m deep is to be made. Determine the 
cost of iron sheet used at the rate of Rs. 5 per metre sheet, sheet being 2 m wide. 
Solution: 
Given, 
Length of tank, l = 12 m 
Width of tank, b = 9m 
Depth of tank, h = 4m 
 
Area of sheet required = total surface area of tank 
                                    = 2 (lb×bh×hl) 
                                    = 2 (12×9 + 9×4 + 4×12) 
                                    = 2 (108 + 36 + 48) 
                                    = 2 (192) 
                                    = 384 m
2
  
Let length be l
1
 
Page 4


 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
EXERCISE 21.4                                                 PAGE NO: 21.30 
 
1. Find the length of the longest rod that can be placed in a room 12 m long, 9 m 
broad and 8 m high. 
Solution: 
Given details are, 
Length of room = 12 m 
Breadth of room = 9m 
Height of room = 8m 
So, 
Length of longest rod that can be placed in room = diagonal of room (cuboid)  
                                                                               = v(l
2
 + b
2
 + h
2
) 
                                                                               = v(12
2
 + 9
2
 + 8
2
) 
                                                                               = v(144+81+64) 
                                                                               = v(289) 
                                                                               = 17m 
 
2. If V is the volume of a cuboid of dimensions a, b, c and S is its surface area, then 
prove that 1/V = 2/S (1/a + 1/b + 1/c) 
Solution: 
Let us consider,  
V = volume of cuboid 
S = surface area of cuboid 
Dimensions of cuboid = a, b, c 
So, 
S = 2 (ab + bc + ca)  
V = abc 
S/V = 2 (ab + bc + ca) / abc 
       = 2[(ab/abc) + (bc/abc) + (ca/abc)] 
       = 2 (1/a + 1/b + 1/c) 
1/V = 2/S (1/a + 1/b + 1/c) 
Hence proved. 
 
3. The areas of three adjacent faces of a cuboid are x, y, and z. If the volume is V, 
prove that V
2
 = xyz. 
Solution: 
Let us consider,  
Areas of three faces of cuboid as x,y,z  
So, Let length of cuboid be = l 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth of cuboid be = b 
Height of cuboid be = h 
Let, x = l×b 
y = b×h 
z = h×l 
Else we can write as 
xyz = l
2
 b
2
 h
2
….. (i) 
 
If ‘V’ is volume of cuboid = V = lbh  
V
2
 = l
2
 b
2
 h
2
 = xyz …… from (i) 
? V
2
 = xyz 
Hence proved. 
 
4. A rectangular water reservoir contains 105 m
3
 of water. Find the depth of the 
water in the reservoir if its base measures 12 m by 3.5 m. 
Solution: 
Given details are, 
Capacity of water reservoir = 105 m
3
 
Length of base of reservoir = 12 m 
Width of base = 3.5 m 
Let the depth of reservoir be ‘h’ m 
l × b × h = 105 
h = 105 / (l × b) 
   = 105 / (12×3.5) 
   = 105/42 
   = 2.5m 
? Depth of reservoir is 2.5 m 
 
5. Cubes A, B, C having edges 18 cm, 24 cm and 30 cm respectively are melted and 
moulded into a new cube D. Find the edge of the bigger cube D. 
Solution: 
Given details are, 
Edge length of cube A = 18 cm 
Edge length of cube B = 24 cm 
Edge length of cube C = 30 cm 
Then, 
Volume of cube A = v
1
 = 18
3
 = 5832cm
3
 
Volume of cube B = v
2
 = 24
3
 = 13824cm
3
  
Volume of cube C = v
3
 = 30
3
 = 27000cm
3
 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Total volume of cube A,B,C = 5832 + 13824 + 27000 = 46656 cm
3
 
 
Let ‘a’ be the length of edge of newly formed cube. 
a
3
 = 46656 
a =
 
3
v(46656) 
   = 36 
? Edge of bigger cube is 36cm 
 
6. The breadth of a room is twice its height, one half of its length and the volume of 
the room is 512 cu. Dm. Find its dimensions. 
Solution: 
Given, 
Breadth of room is twice of its height, b = 2h or h = b/2 … (i) 
Breadth is one half of length, b = l/2 or l = 2b … (ii)  
Volume of the room = lbh = 512 dm
3
 … (iii) 
By substituting (i) and (ii) in (iii) 
2b × b × b/2 = 512 
b
3
 = 512 
b = 
3
v(512) 
   = 8 
? Breadth of cube = b = 8 dm 
Length of cube = 2b = 2×8 = 16 dm 
Height of cube = b/2 = 8/2 = 4 dm  
 
7. A closed iron tank 12 m long, 9 m wide and 4 m deep is to be made. Determine the 
cost of iron sheet used at the rate of Rs. 5 per metre sheet, sheet being 2 m wide. 
Solution: 
Given, 
Length of tank, l = 12 m 
Width of tank, b = 9m 
Depth of tank, h = 4m 
 
Area of sheet required = total surface area of tank 
                                    = 2 (lb×bh×hl) 
                                    = 2 (12×9 + 9×4 + 4×12) 
                                    = 2 (108 + 36 + 48) 
                                    = 2 (192) 
                                    = 384 m
2
  
Let length be l
1
 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth be b
1
 
Given, b
1
 = 2m 
l
1
 × b
1
 = 384 
l
1
 = 384/b
1
 
   = 384/2 
   = 192m 
? Cost of iron sheet at the rate of Rs 5 per metre = 5 × 192 = Rs 960 
 
8. A tank open at the top is made of iron sheet 4 m wide. If the dimensions of the 
tank are 12m×8m×6m, find the cost of iron sheet at Rs. 17.50 per metre. 
Solution: 
Given details are, 
Dimensions of tank = 12m × 8m × 6m 
Where, length = 12m 
Breadth = 8m 
Height = 6m 
Area of sheet required = total surface area of tank with one top open 
                                    = l × b + 2 (l×h + b×h) 
                                    = 12 × 8 + 2 (12×6 + 8×6) 
                                    = 96 + 240 
                                    = 336 m
2
 
Let length be l
1
 
Breadth be b
1
 
Given, b
1
 = 4m 
l
1
 × b
1
 = 336 
l
1
 = 336/b
1
 
   = 336/4 
   = 84m 
? Cost of iron sheet at the rate of Rs 17.50 per metre = 17.50 × 84 = Rs 1470 
 
9. Three equal cubes are placed adjacently in a row. Find the ratio of total surface 
area of the new cuboid to that of the sum of the surface areas of the three cubes. 
Solution: 
Given details are, 
Let edge length of three equal cubes = a 
Then, 
Sum of surface area of 3 cubes = 3 × 6a
2
 = 18a
2
  
When these cubes are placed in a row adjacently they form a cuboid. 
Length of new cuboid formed = a + a + a = 3a 
Page 5


 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
EXERCISE 21.4                                                 PAGE NO: 21.30 
 
1. Find the length of the longest rod that can be placed in a room 12 m long, 9 m 
broad and 8 m high. 
Solution: 
Given details are, 
Length of room = 12 m 
Breadth of room = 9m 
Height of room = 8m 
So, 
Length of longest rod that can be placed in room = diagonal of room (cuboid)  
                                                                               = v(l
2
 + b
2
 + h
2
) 
                                                                               = v(12
2
 + 9
2
 + 8
2
) 
                                                                               = v(144+81+64) 
                                                                               = v(289) 
                                                                               = 17m 
 
2. If V is the volume of a cuboid of dimensions a, b, c and S is its surface area, then 
prove that 1/V = 2/S (1/a + 1/b + 1/c) 
Solution: 
Let us consider,  
V = volume of cuboid 
S = surface area of cuboid 
Dimensions of cuboid = a, b, c 
So, 
S = 2 (ab + bc + ca)  
V = abc 
S/V = 2 (ab + bc + ca) / abc 
       = 2[(ab/abc) + (bc/abc) + (ca/abc)] 
       = 2 (1/a + 1/b + 1/c) 
1/V = 2/S (1/a + 1/b + 1/c) 
Hence proved. 
 
3. The areas of three adjacent faces of a cuboid are x, y, and z. If the volume is V, 
prove that V
2
 = xyz. 
Solution: 
Let us consider,  
Areas of three faces of cuboid as x,y,z  
So, Let length of cuboid be = l 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth of cuboid be = b 
Height of cuboid be = h 
Let, x = l×b 
y = b×h 
z = h×l 
Else we can write as 
xyz = l
2
 b
2
 h
2
….. (i) 
 
If ‘V’ is volume of cuboid = V = lbh  
V
2
 = l
2
 b
2
 h
2
 = xyz …… from (i) 
? V
2
 = xyz 
Hence proved. 
 
4. A rectangular water reservoir contains 105 m
3
 of water. Find the depth of the 
water in the reservoir if its base measures 12 m by 3.5 m. 
Solution: 
Given details are, 
Capacity of water reservoir = 105 m
3
 
Length of base of reservoir = 12 m 
Width of base = 3.5 m 
Let the depth of reservoir be ‘h’ m 
l × b × h = 105 
h = 105 / (l × b) 
   = 105 / (12×3.5) 
   = 105/42 
   = 2.5m 
? Depth of reservoir is 2.5 m 
 
5. Cubes A, B, C having edges 18 cm, 24 cm and 30 cm respectively are melted and 
moulded into a new cube D. Find the edge of the bigger cube D. 
Solution: 
Given details are, 
Edge length of cube A = 18 cm 
Edge length of cube B = 24 cm 
Edge length of cube C = 30 cm 
Then, 
Volume of cube A = v
1
 = 18
3
 = 5832cm
3
 
Volume of cube B = v
2
 = 24
3
 = 13824cm
3
  
Volume of cube C = v
3
 = 30
3
 = 27000cm
3
 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Total volume of cube A,B,C = 5832 + 13824 + 27000 = 46656 cm
3
 
 
Let ‘a’ be the length of edge of newly formed cube. 
a
3
 = 46656 
a =
 
3
v(46656) 
   = 36 
? Edge of bigger cube is 36cm 
 
6. The breadth of a room is twice its height, one half of its length and the volume of 
the room is 512 cu. Dm. Find its dimensions. 
Solution: 
Given, 
Breadth of room is twice of its height, b = 2h or h = b/2 … (i) 
Breadth is one half of length, b = l/2 or l = 2b … (ii)  
Volume of the room = lbh = 512 dm
3
 … (iii) 
By substituting (i) and (ii) in (iii) 
2b × b × b/2 = 512 
b
3
 = 512 
b = 
3
v(512) 
   = 8 
? Breadth of cube = b = 8 dm 
Length of cube = 2b = 2×8 = 16 dm 
Height of cube = b/2 = 8/2 = 4 dm  
 
7. A closed iron tank 12 m long, 9 m wide and 4 m deep is to be made. Determine the 
cost of iron sheet used at the rate of Rs. 5 per metre sheet, sheet being 2 m wide. 
Solution: 
Given, 
Length of tank, l = 12 m 
Width of tank, b = 9m 
Depth of tank, h = 4m 
 
Area of sheet required = total surface area of tank 
                                    = 2 (lb×bh×hl) 
                                    = 2 (12×9 + 9×4 + 4×12) 
                                    = 2 (108 + 36 + 48) 
                                    = 2 (192) 
                                    = 384 m
2
  
Let length be l
1
 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth be b
1
 
Given, b
1
 = 2m 
l
1
 × b
1
 = 384 
l
1
 = 384/b
1
 
   = 384/2 
   = 192m 
? Cost of iron sheet at the rate of Rs 5 per metre = 5 × 192 = Rs 960 
 
8. A tank open at the top is made of iron sheet 4 m wide. If the dimensions of the 
tank are 12m×8m×6m, find the cost of iron sheet at Rs. 17.50 per metre. 
Solution: 
Given details are, 
Dimensions of tank = 12m × 8m × 6m 
Where, length = 12m 
Breadth = 8m 
Height = 6m 
Area of sheet required = total surface area of tank with one top open 
                                    = l × b + 2 (l×h + b×h) 
                                    = 12 × 8 + 2 (12×6 + 8×6) 
                                    = 96 + 240 
                                    = 336 m
2
 
Let length be l
1
 
Breadth be b
1
 
Given, b
1
 = 4m 
l
1
 × b
1
 = 336 
l
1
 = 336/b
1
 
   = 336/4 
   = 84m 
? Cost of iron sheet at the rate of Rs 17.50 per metre = 17.50 × 84 = Rs 1470 
 
9. Three equal cubes are placed adjacently in a row. Find the ratio of total surface 
area of the new cuboid to that of the sum of the surface areas of the three cubes. 
Solution: 
Given details are, 
Let edge length of three equal cubes = a 
Then, 
Sum of surface area of 3 cubes = 3 × 6a
2
 = 18a
2
  
When these cubes are placed in a row adjacently they form a cuboid. 
Length of new cuboid formed = a + a + a = 3a 
 
 
 
 
RD Sharma Solutions for Class 8 Maths Chapter 21 – Mensuration – 
II (Volumes and Surface Areas of a Cuboid and a Cube) 
 
Breadth of cuboid = a 
Height of cuboid = a 
Total surface area of cuboid = 2 (lb×bh×hl) 
                                              = 2 (3a×a + a×a + a×3a) 
                                              = 2 (3a
2
 + a
2
 + 3a
2
) 
                                              = 2 (7a
2
) 
                                              = 14 a
2
  
Total surface area of new cuboid / sum of surface area of 3 cuboids = 14/18 = 7/9 = 7:9 
? The ratio is 7:9 
 
10. The dimensions of a room are 12.5 m by 9 m by 7 m. There are 2 doors and 4 
windows in the room; each door measures 2.5 m by 1.2 m and each window 1.5 m by 
1 m. Find the cost of painting the walls at Rs. 3.50 per square metre. 
Solution: 
Given details are, 
Dimensions of room = 12.5m × 9m × 7m 
Dimensions of each door = 2.5m × 1.2m 
Dimensions of each window = 1.5m × 1m 
 
Area of four walls including doors and windows = 2 (l×h + b×h) 
                                                                              = 2 (12.5×7 + 9×7) 
                                                                              = 2 (87.5 + 63) 
                                                                              = 2 (150.5) 
                                                                              = 301 m
2
 
 
Area of 2 doors and 4 windows = 2 (2.5×1.2) + 4 (1.5×1) 
                                                   = 2(3) + 4 (1.5) 
                                                   = 6 + 6 
                                                   = 12 m
2
 
 
Area of only walls = 301 – 12  
                              = 289 m
2
 
? Cost of painting the walls at the rate of Rs 3.50 per square metre = Rs (3.50 × 289) = 
Rs 1011.50 
 
11. A field is 150m long and 100m wide. A plot (outside the field) 50m long and 30m 
wide is dug to a depth of 8m and the earth taken out from the plot is spread evenly 
in the field. By how much is the level of field raised? 
Solution: 
Read More
79 videos|408 docs|31 tests

Top Courses for Class 8

79 videos|408 docs|31 tests
Download as PDF
Explore Courses for Class 8 exam

Top Courses for Class 8

Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Related Searches

Mensuration - II (Volumes and Surface Areas of a Cuboid and a Cube) - EXERCISE 21.4 | Mathematics (Maths) Class 8

,

Mensuration - II (Volumes and Surface Areas of a Cuboid and a Cube) - EXERCISE 21.4 | Mathematics (Maths) Class 8

,

study material

,

Extra Questions

,

MCQs

,

past year papers

,

Free

,

shortcuts and tricks

,

Sample Paper

,

Important questions

,

pdf

,

Previous Year Questions with Solutions

,

ppt

,

practice quizzes

,

mock tests for examination

,

Viva Questions

,

Objective type Questions

,

Mensuration - II (Volumes and Surface Areas of a Cuboid and a Cube) - EXERCISE 21.4 | Mathematics (Maths) Class 8

,

Summary

,

Exam

,

Semester Notes

,

video lectures

;