BOATS and STREAMS Q75

75. In a stream running at 2 kmph, a motorboat goes 6 km upstream and back again to the starting point in 33 minutes. Find the speed of the motorboat in still water.

A.)    12 kmph

B.)    15 kmph

C.)    22 kmph

D.)    30 kmph 

Answer: Option A

Explanation:

Let the speed of the motorboat in still water be x kmph. Then

Speed downstream = (x+2) kmph; Speed upstream = (x-2) kmph

 6/x+2 + 6/x-2 = 33/60

11x² – 240x – 44 =0

11x² – 242x + 2x – 44 =0

(x - 22) (11x + 2) = 0

= x= 22

Hence, speed of motorboat in still water = 22 kmph