A motor boat goes 10 km up stream and comes back again to the starting point in 55 min The current o...........

Quantitative Aptitude ( CCS) Boats and Streams

A motor boat goes 10 km up stream and comes back again to the starting point in 55 min. The current of a stream runs at the rate of 2 km per hr. Find the speed of the motor boat in still water ?

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Explanation:

Let the speed of boat in still water be x kmph
Speed downward = (x + 2) kmph
Speed upstream = (x – 2) kmph
Time taken to row 10 kms and back = 10/(x + 2) + 10/(x - 2) hrs
10/(x + 2) + 10/(x - 2) = 55/60 (or) 11x2 - 240x - 44 = 0
(or) 11x2 - 242x + 2x - 44 = 0 (or) (x - 22) (11x + 2) = 0
x = 22
Hence speed of motor boat in still water is 22 kmph