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1. A man's speed with the current is 15 km/hr and the speed of the current is 2.5 km/hr. The man's speed against the current is:
A. 8.5 km/hr
B. 10 km/hr.
C. 12.5 km/hr
D. 9 km/hr
Answer: Option B
Explanation:
Man's speed with the current = 15 km/hr
=> speed of the man + speed of the current = 15 km/hr
speed of the current is 2.5 km/hr
Hence, speed of the man = 15  2.5 = 12.5 km/hr
man's speed against the current = speed of the man  speed of the current
= 12.5  2.5 = 10 km/hr
2. A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:
A. 10
B. 6
C. 5
D. 4
Answer: Option C
Explanation:
Speed of the motor boat =15 km/hr
Let speed of the stream =v
Speed downstream =(15+v) km/hr
Speed upstream =(15−v) km/hr
3. In one hour, a boat goes 14 km/hr along the stream and 8 km/hr against the stream. The speed of the boat in still water (in km/hr) is:
A. 12 km/hr
B. 11 km/hr
C. 10 km/hr
D. 8 km/hr
Answer: Option B
Explanation:
Let speed of the boat in still water = a and speed of the stream = b
Then
a + b = 14
a  b = 8
Adding these two equations, we get 2a = 22
=> a = 11
ie, speed of boat in still water = 11 km/hr
4. A man rows to a place 48 km distant and come back in 14 hours. He finds that he can row 4 km with the stream in the same time as 3 km against the stream. The rate of the stream is:
A. 1 km/hr.
B. 2 km/hr.
C. 1.5 km/hr.
D. 2.5 km/hr.
Answer: Option A
Explanation:
Assume that he moves 4 km downstream in x hours
Given that he can row 4 km with the stream in the same time as 3 km against the stream
i.e., speed upstream =3/4 of speed downstream
=> speed upstream = 3x km/hr
He rows to a place 48 km distant and comes back in 14 hours
Now we can use the below formula to find the rate of the stream
Hence, rate of the stream = 1/2(8−6) = 1km/hr
5. A boatman goes 2 km against the current of the stream in 2 hour and goes 1 km along the current in 20 minutes. How long will it take to go 5 km in stationary water?
A. 2 hr 30 min
B. 2 hr
C. 4 hr
D. 1 hr 15 min
Answer: Option A
Explanation:
= 2 hour 30 minutes
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