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

1. 4
2. 5
3. 6
4. 10

Correct answer: 5

Solution

Let stream speed be \(x\) km/h. Then downstream speed is \(15+x\) and upstream speed is \(15-x\). Using total time \(4.5\) hours, \(\frac{30}{15+x}+\frac{30}{15-x}=4.5\), which gives \(x=5\).

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