Multiple choice

A motor boat can travel at 8 km/hour in still water. It travelled 60 km down stream in a river and then returned to the same place in 20 hours. Find the rate of flow of river.

  1. 2 km/h

  2. 3 km/h

  3. 4 km/h

  4. 3.5 km/h
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Speed downstream = 8 + v, upstream = 8 - v where v is river speed. Time = 60/(8+v) + 60/(8-v) = 20 hours. Solving: 60(8-v + 8+v)/((8+v)(8-v)) = 20. 60×16/(64-v²) = 20. 960 = 20(64-v²). 48 = 64 - v². v² = 16, v = 4 km/h.

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