A person can row 7 1 2 km an hour in still water and he finds that it takes him twice as long t...........

Quant (SSC) Boats and Streams

A person can row 7 1/2 km an hour in still water and he finds that it takes him twice as long to row up as to row down the river. The speed of the stream is:

2 km/hr

3 km/hr

2 1/2 km/hr

3 1/2 km/hr

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Explanation:

x(n - 1 /n + 1) km/hr
Given,
x = Person speed = 7 1/2 = 15/2 km
n = the no. of times of hours taken by the boat in the 2nd scenario (upstream) = 2
By the short trick approach, we get
15/2 (2 - 1 /2 + 1)
= 15/2 x 1/3 = 2 1/2 km/hr