A motor-boat can travel at 10 km/hour in
still water. It travelled 91 km downstream
in a river and then returned to the same
place, taking altogether 20 hours. Find
the rate of flow of river.

• Given , speed of boat = 10 km/h
  Let speed of flow of river = x km/h
  Therefore Upstream speed of boat = (10 – x )km/h and downstream speed of boat
  According to question,
  = (10 + x) km/h
  91 / (10 - x) + 91 / (10 + x) = 20
  = 91(10 + x + 10 –x ) / ( 10 – x) ( 10 + x) = 20
  = 91 (20) / 100 – x2 = 20
  = 91 = 100 – x2
  = x2 = 9
  Therefore x = 3
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