• Let V be the speed of the stream. Then the boat's speed downstream is 36+V and upstream is 36-V. Although we don't know the time duration of each leg of the trip, since the time is the same we know that distance is proportional to speed. So we can set up a ratio: (36+v) / (36-V) = 50 / 30 = 5/3 3*(36+V) = 5*(36-V) 3V + 108 = -5V + 180 8V = 180-108 = 72 V = 9 The stream flows at 9 mph.
  • The boat travels 80 miles. Now consider Bob who is on a raft floating with the stream at the point the boat starts out. Bob sees the boat leave going at a constant speed, turn round and travel the same distance back, relative to Bob. For this journey back to be 20 miles less relative to the stationary bank. Bob must have moved 20 miles in the same time that the other boat went 80. He's travelling with the stream at 36/4 = 9mph.

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