boats and streams aptitude questions

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

 

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