Explanatory Answer

Let the speed of the boat in still water be ks and the speed of the stream be s

If d is the length of the one way, then time taken is

d/(ks+s) + d/(ks-s) = 2kd/((k^2-1) s)

if speed of boat in still water doubles, then

time taken would be 4kd/((4k^2-1) s)

The ratio of above two expressions is 8:3.2

Hence dividing the two

(4k^2 – 1)/ (k^2 – 1) = 5 solving for k, we get k^2 = 4 and hence k = 2, hence ratio is 2: 1. 

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