A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and

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Question:  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:

Options are:

A.     4
B.     5
C.     6
D.     10

Correct answer is:

B.     5