A man takes 20 minutes to row 12 km upstream which is a third more than the time he takes on his way...........

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A man takes 20 minutes to row 12 km upstream which is a third more than the time he takes on his way downstream. What is his speed in still water?

41 km/hr

36 km/hr

42 km/hr

45 km/hr

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Explanation:

Let the speed in still water = x km/hr.
Takes 20 min. to row 12 km upstream ⇒ speed of u/s = 36 km/hr.
Also, time taken for u/s is 1/3 more than for d/s.
∴ distance covered in d / s will be 1/3 more.
Hence distance covered by man for d / s in 20 min. = 12 × (12/3) = 16km.
So speed of d / s = 48 km/hr.
∴ x + y = 48 and x – y = 36 ⇒ x = 42 km/hr.