Tips, Tricks and Shortcuts for Boats and Streams Questions

Type 3: Using Man’s Still Water Speed Calculate Stream’s Speed

Question 1:A sailor can row 6 km/h in still water. It takes him twice as long to row up as to row down the river. Find the speed of the stream.

Options:

A. 6 km/ hr

B. 5 km/ hr

C. 3 km/ hr

D. 2 km/ hr

Solution:     Let sailor’s speed in upstream = x

Downstream speed = 2x (As given in the question, his downstream speed is twice of upstream speed)

Man’s speed in still water = 1/2 (Upstream speed + Downstream speed)

= 1/2 (x + 2x) = 3x/2

3x/2 = 6 (sailor can row 6 km/h in still water)

x = 4 km/hr

Therefore, upstream speed = x = 4 km/hr

Downstream speed = 2x = 2 * 4 = 8 km/hr

Speed of stream = 1/2*  (Downstream speed – Upstream speed)

= 1/2 (8 – 4)

= 2 km/ hr

Correct option: D