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Question

A man went downstream for 28 km in a motor boat and immediately returned. It took the man twice as long to make the return trip. If the speed of the river flow were twice as high, the trip downstream and back would take 672 minutes. Find the speed of the boat in still water and the speed of the river flow.

A) 12 km/hr, 3 km/hr	B) 9 km/hr, 3 km/hr
C) 8 km/hr, 2 km/hr	D) 9 km/hr, 6 km/hr

Answer: B) 9 km/hr, 3 km/hr

Explanation:

Let the speed of the boat = p kmph

Let the speed of the river flow = q kmph

From the given data,

$2 x \frac{28}{p + q} = \frac{28}{p - q}$

=> 56p - 56q -28p - 28q = 0

=> 28p = 84q

=> p = 3q.

Now, given that if

$\frac{28}{3 q + 2 q} + \frac{28}{3 q - 2 q} = \frac{672}{60} = > \frac{28}{5 q} + \frac{28}{q} = \frac{672}{60} = > q = 3 kmph = > x = 3 q = 9 kmph$

Hence, the speed of the boat = p kmph = 9 kmph and the speed of the river flow = q kmph = 3 kmph.

Subject: Boats and Streams
Exam Prep: AIEEE , Bank Exams , CAT , GATE
Job Role: Analyst , Bank Clerk , Bank PO

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