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All questions of Mensuration for UGC NET Exam

Cylindrical cans of cricket balls are to be packed in a box. Each can has a radius of 7 cm and height of 30 cm. Dimension of the box is l = 76 cm, b = 46 cm, h = 45 cm. What is the maximum number of cans that can fit in the box?
  • a)
    15
  • b)
    17
  • c)
    22
  • d)
    21
Correct answer is option 'D'. Can you explain this answer?

Naroj Boda answered
This question requires a good deal of visualization. Since, both the box and cans are hard solids, simply dividing the volume won’t work because the shape can’t be deformed. 

Each cylindrical can has a diameter of 14 cm and while they are kept erect in the box will occupy height of 30 cm 

Number of such cans that can be placed in a row

Number of such rows that can be placed

Thus 5 x 3 = 15 cans can be placed in an erect position.

However, height of box = 45cm and only 30 cm has been utilized so far 

Remaining height = 15 cm > 14 cm (Diameter of the can)

So, some cans can be placed horizontally on the base.

Number of cans in horizontal row
Number of such rows
∴ 2 x 3 = 6 cans can be placed horizontally

∴ Maximum number of cans = 15+6 = 21 

Choice (D) is therefore, the correct answer.
 
 
 
 

PQRST is a pentagon in which all the interior angles are unequal. A circle of radius ‘r’ is inscribed in each of the vertices. Find the area of portion of circles falling inside the pentagon. 
  • a)
    πr2
  • b)
    1.5πr2
  • c)
    2πr2
  • d)
    1.25πr2
Correct answer is option 'B'. Can you explain this answer?

Preeti Khanna answered
Since neither angles nor sides are given in the question, immediately the sum of angles of pentagon should come in mind. To use it,

We know the area of the sectors of a circle is given as,
Note => The above concept is applicable for a polygon of n sides.

Choice (B) is therefore, the correct answer.

Correct Answer: 1.5πr2
 
 

PQRS is a circle and circles are drawn with PO, QO, RO and SO as diameters. Areas A and B are marked. A/B is equal to:
  • a)
    π
  • b)
    1
  • c)
    π/4
  • d)
    2
Correct answer is option 'B'. Can you explain this answer?

Divey Sethi answered
Such questions are all about visualization and ability to write one area in terms of others.

Here, Let the radius of PQRS be 2r 
∴ Radius of each of the smaller circles = 2r/2 = r

Area A can be written as:
A = π (2r)2 – 4 x π(r)2 (Area of the four smaller circles) + B (since, B has been counted twice in the previous subtraction)
A = 4πr2 - 4πr2 + B
A = B
A/B = 1
Choice (B) is therefore, the correct answer.
Correct Answer: 1

Figure above shows a box which has to be completely wrapped with paper. However, a single Sheet of paper need to be used without any tearing. The dimension of the required paper could be 
  • a)
    17 cm by 4 cm
  • b)
    12 cm by 6 cm
  • c)
    15 cm by 4 cm
  • d)
    13 cm by 4 cm
Correct answer is option 'B'. Can you explain this answer?

Naroj Boda answered
Total surface area of the box = 2(4x6 + 1x6 + 1x4)

= 2(24 + 6 + 4)

= 68 cm2

As the problem says the paper can’t be torn/cut a portion of paper will need to be fold, so, the area of paper required would be greater than 68 cm2. Only option b) gives the area greater 68 cm2

Choice (B) is therefore, the correct answer.

Correct Answer: 12 cm by 6 cm

A right circular cone has height H and radius R. A small cone is cut off at the top by a plane parallel to the base. At what height above the base the section has been made?

Statement (I): H = 20 cm
Statement (II): Volume of small cone: volume of large cone : 1:15
  • a)
    If the question can be answered with statement I alone but not statement II alone, or can be answered with statement II alone but not statement I alone.
  • b)
    If the question cannot be answered with statement I alone or with statement II alone, but can be answered if both statements are used together.
  • c)
    If the question can be answered with either statement alone.
  • d)
    If the question cannot be answered with the information provided.
Correct answer is option 'B'. Can you explain this answer?

Ishani Rane answered
From statement I, we know that the height of the initial cone is 20cm. However, nothing is said about the small cone. Hence, we cannot answer the question using statement A. So, we can eliminate choices (A) and (D).

We are down to choices (A), (B) or (D).

From Statement II, we know that the ratio of the volume of the small cone to that of the large cone is 1 : 15.

i.e. *π*r2*h : *π*R2*H is 1 : 15 (r is the base radius of the smaller cone and h is the height of the smaller cone)
or r2 * h : R2 * H is 1 : 15

From this information, we will not be able to find the answer to h. Hence, we can eliminate choice (A). 

Combining the information in the two statements: 

When a section is made the two cones are similar triangles. so = 
R = 

We know H = 20 
h = * r

i.e., h3 = H3. Substituting H = 20, we can get the value for h.
Choice (B) is therefore, the correct answer.

Correct Answer: If the question cannot be answered with statement I alone or with statement II alone, but can be answered if both statements are used together.

Anil grows tomatoes in his backyard which is in the shape of a square. Each tomato takes 1 cm2 in his backyard. This year, he has been able to grow 131 more tomatoes than last year. The shape of the backyard remained a square. How many tomatoes did Anil produce this year?
  • a)
    4225
  • b)
    4096
  • c)
    4356
  • d)
    Insufficient Data
Correct answer is option 'C'. Can you explain this answer?

Naveen Jain answered
Let the area of backyard be x2 this year and y2 last year

∴ X2- Y2 = 131

=) (X+Y) (X-Y) = 131

Now, 131 is a prime number (a unique one too. Check out its properties on Google). Also, always identify the prime number given in a question. Might be helpful in cracking the solution.

=) (X+Y) (X-Y) = 131 x 1

=) X+Y = 131

X-Y = 1

=) 2X = 132 =) X = 66 

and Y = 65

∴ Number of tomatoes produced this year = 662 = 4356

Choice (C) is therefore, the correct answer.

Correct Answer: 4356

A square PQRS has an equilateral triangle PTO inscribed as shown:
What is the ratio of AΔPQT to AΔTRU?
  • a)
    1 : 3
  • b)
    1 : √3
  • c)
    1 : √2
  • d)
    1 : 2
Correct answer is option 'D'. Can you explain this answer?

Let PQ, a side of equilateral triangle be b


By symmetry QT=ST=z (say)


=) a^2 + z^2 – 2az = 2az (Please note how the solution is being managed here. You must always be aware of what you are looking for. Here, as equation -℗ we are looking for (a-z)2 in terms of az) 

PQRS is a square of sides 2 cm & ST = 2 cm. Also, PT=RT. What is the area of ?PST?
  • a)
    2 cm2
  • b)
    √3 cm2
  • c)
    √2 cm2
  • d)
Correct answer is option 'C'. Can you explain this answer?

Rajeev Kumar answered
Although the figure looks like a 3D figure but on reading through the question, it is clear that the diagram is on a single plane.

 
 
 

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