The speed of a motor-boat in still water and the speed of the current of water are in the ratio 20 : 4. To go a certain distance along the current the boat takes 4 hours. Find the time taken by the boat to cover the same distance against the current.

6 hours

Ratio of Boat to stream is 20 : 4

Along  the current , Downstream speed = (20 + 4)R = 24R

Time to cover Distance = 4 hours

Distance = Speed × Time

= 24R ×4 = 96R

Against  the current , Upstream speed = (20 - 4)R = 16R

Time to cover distance against the current = \(\frac{distance}{Speed}\)

= \(\frac{96R}{16R}\) = 6 hours