Question

A man travels by boat 36 km down a river and back in 8 hours.
If the speed of the boat down the current is 12 km/h, find the speed of the river current​

Answer 1

a man travels by boat 36 km down a river and back in 8 hrs and the speed of his boat in still water is 12km/h

so, Distance, d = 36km

Total time taken for down a river and back in 8hrs

so, Time of downstream + Time of upstream = 8.................(1)

Speed of his boat in still water is 12km/hr

Speed of stream or river = x km/h

so, Speed of downstream = speed of boat in still water + speed of river = (12 + x) km/h

Speed of upstream = Speed of boat in still water - speed of river = (12 - x) km/h

so, Time of downstream = Distance / Speed of downstream = 36/(12+x)................(2)

Time of upstream = Distance / speed of upstream = 36/(12-x).......................(3)

substitute (2) and (3) in (1)

so the speed of the river = 6km/h

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