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:

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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:
[A].

4

[B].

5

[C].

6

[D].

10

Answer: Option B

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

9×2 = 225

x2 = 25

x = 5 km/hr.

Video Explanation: https://youtu.be/lMFnNB3YQOo