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Boats and Streams - General Aptitude

Q1:

A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:

A 4

B 5

C 6

D 10

ANS:B - 5

Let the speed of the stream be x km/hr. Then, Speed downstream = (15 + x) km/hr, Speed upstream = (15 - x) km/hr.

30 + 30 = 4 1
(15 + x) (15 - x) 2
900 = 9
225 - x2 2
 9x2 = 225  x2 = 25  x = 5 km/hr.