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:

4
5
6
10

Correct Answer: 5

Explanation:

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.

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