A motorboat in still water travels at speed of 36 kmph. It goes 56 km upstream in 1 hour 45 minutes. The time taken by it to cover the same distance down the stream will be:

Explanation:-

1 hour 45 minutes = 1+(⁴⁵/₆₀) = ⁷/₄ hours.
Speed of the motorboat up-stream,
= Distance /time taken
= ^{56 km}/_{(7/4) h} = ^{56 * 4}/₇ = 32 kmph
Let the speed of the current = x kmph
Hence, 36-x = 32
Or, x = 36-32 = 4 kmph
Speed of boat down the stream = 36 + 4 = 40 kmph.