Boats and Streams

Question:
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: 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.

Question: 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: 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.

Question: A boat running downstream covers a distance of 16 km in 2 hours while for covering the same distance upstream, it takes 4 hours. What is the speed of the boat in still water?
[A].

4 km/hr

[B].

6 km/hr

[C].

8 km/hr

[D].

Data inadequate

Answer: Option B

Explanation:

Rate downstream =		16	kmph = 8 kmph.
		2

Rate upstream =		16	kmph = 4 kmph.
		4

Speed in still water =	1	(8 + 4) kmph = 6 kmph.
	2

Video Explanation: https://youtu.be/3D8uAfzz3l0

Question: In one hour, a boat goes 11 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is:
[A].

3 km/hr

[B].

5 km/hr

[C].

8 km/hr

[D].

9 km/hr

Answer: Option C

Explanation:

Speed in still water =	1	(11 + 5) kmph = 8 kmph.
	2

Video Explanation: https://youtu.be/KQX_mA3tcVA