A man takes half time in rowing a certain distance downstream than upstream What is the ratio or the...........

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A man takes half time in rowing a certain distance downstream than upstream. What is the ratio or the speed in still water to the speed of current?

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Explanation:

Sol. Ans.(d).

Regular method: Let the speed in still water = s kmph and the speed of current = c kmph. Let distance is D km.

Speed upstream = Speed in still water + Speed of current = s + c.

Speed downstream = Speed in still water – Speed of current = s – c.

Time upstream = Distance/Speed upstream = D/(s –c) …(1) and

Time downstream = Distance/Speed downstream = D/(s+c) …(2)

By the condition given in question Time upstream/2 = Time downstream

Shortcut method: Try working backwards. For option (d), s = 3x, c = 1x. So downstream speed = 4x and upstream speed = 2x. Clearly for the same distance, ratio of times taken will be ratio of these speeds, and that comes out to 4x : 2x = 2:1. (upstream time : downstream time). Hence option (d) is correct.