Question 43

A boat takes 4 hours to travel from P to Q upstream. If the distance between P and Q is 20 km and the speed of the boat in still water is 3 times the speed of the stream, then how much time will the boat take to travel from Q to P downstream?

Let the speed of the boat be 3x and the speed of the stream be x.
Then it is given that
20/(3x-x) = 4
=> 2x = 5
=> x = 2.5
=> Speed of the boat is 7.5 km/hr and speed of the stream is 2.5 km/hr
Now while going downstream, the effective speed would be 7.5 + 2.5 = 10 km/hr
Hence the required time = 20/10 = 2 hours.

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