11.
A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will it take to go 5 km in stationary water?
A. 40 minutes B. 1 hour
C. 1 hr 15 min D. 1 hr 30 min
Answer & Explanation
Answer: Option C
Explanation:
Rate downstream = 1 x 60 km/hr = 6 km/hr.
10
Rate upstream = 2 km/hr.
Speed in still water = 1 (6 + 2) km/hr = 4 km/hr.
2
Required time = 5 hrs = 1 1 hrs = 1 hr 15 min.
4 4
12.
A man can row three-quarters of a kilometre against the stream in 11 minutes and down the stream in 7 minutes. The speed (in km/hr) of the man in still water is:
A. 2 B. 3
C. 4 D. 5
Answer & Explanation
Answer: Option D
Explanation:
We can write three-quarters of a kilometre as 750 metres,
and 11 minutes as 675 seconds.
Rate upstream = 750 m/sec = 10 m/sec.
675 9
Rate downstream = 750 m/sec = 5 m/sec.
450 3
Rate in still water = 1 10 + 5 m/sec
2 9 3
= 25 m/sec
18
= 25 x 18 km/hr
18 5
= 5 km/hr.
13.
Speed of a boat in standing water is 9 kmph and the speed of the stream is 1.5 kmph. A man rows to a place at a distance of 105 km and comes back to the starting point. The total time taken by him is:
A. 16 hours B. 18 hours
C. 20 hours D. 24 hours
Answer & Explanation
Answer: Option D
Explanation:
Speed upstream = 7.5 kmph.
Speed downstream = 10.5 kmph.
Total time taken = 105 + 105 hours = 24 hours.
7.5 10.5
14.
A man takes twice as long to row a distance against the stream as to row the same distance in favour of the stream. The ratio of the speed of the boat (in still water) and the stream is:
A. 2 : 1 B. 3 : 1
C. 3 : 2 D. 4 : 3
Answer & Explanation
Answer: Option B
Explanation:
Let man’s rate upstream be x kmph.
Then, his rate downstream = 2x kmph.
(Speed in still water) : (Speed of stream) = 2x + x : 2x – x
2 2
= 3x : x
2 2
= 3 : 1.
15.
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. 1.5 km/hr
C. 2 km/hr D. 2.5 km/hr
Answer & Explanation
Answer: Option A
Explanation:
Suppose he move 4 km downstream in x hours. Then,
Speed downstream = 4 km/hr.
x
Speed upstream = 3 km/hr.
x
48 + 48 = 14 or x = 1 .
(4/x) (3/x) 2
So, Speed downstream = 8 km/hr, Speed upstream = 6 km/hr.
Rate of the stream = 1 (8 – 6) km/hr = 1 km/hr.
2