A man can row 20 kmph in still water and finds that it takes him 4 times as much time to row up than as to row down the same distance in the river. The speed of the current is :
Let speed upstream be x kmph, then speed downstream be = 4x kmph
speed in still water = (4x + x)/2 = 20
=> 5x =40
=> x = 8 kmph.
So, speed upstream = 8 kmph; speed downstream = 4x = 4*8 = 32 kmph
Hence, speed of the current = (32 - 8)/2 = 24/2
= 12 kmph.
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