Quantitative Aptitude :: Boats and Streams - Discussion

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Explanation:
Let's assume Boat speed in still water = x kmph
and current speed = y kmph.

so boat Upstream speed = (x-y) kmph

Boat Downstream speed = (x+y) kmph

as per the question
(x - y) = 15/5 = 3 km/h ---------------(1)

Given x= 4y (speed of current is 1/4 of boat speed in still water)

so form eq. (1 )
4y-y = 3 => y = 1 kmph so x= 4 km/h

So boat downstream speed (x+y) = 5km/h

=> Time required in downstream = 15/5 = 3h