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A motorboat can travel at 10 km/h in still water. It travelled 91km downstream in a river and then returned to the same place, taking altogether 20 h. The rate of flow of river is


A) 3 km/h

B) 4 km/h

C) 2 km/h

D) 5 km/h

Correct Answer:
A) 3 km/h

Description for Correct answer:
Given, speed of boat = 10 km/h

Let speed of flow of river = x km/h

Upstream speed of boat = (10 - x) km/h

and downstream speed of boat

According to question,

= (10 + x) km/h

\( \Large \frac{91}{10-x}+\frac{91}{10+x}=20 \)

\( \Large \frac{91 \left(10+x+10-x\right) }{ \left(10-x\right) \left(10+x\right) } = 20 \)

\( \Large \frac{91 \left(20\right) }{100-x^{2}} = x^{2}-9 \)

x = 3

Part of solved Boat and Stream questions and answers : >> Aptitude >> Boat and Stream




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