A motorboat whose speed in still water is 10 km/h went 91 km downstream and then returned to its starting point. Calculate the speed of the river flow if the round trip took of 20 hours.

3 km/h

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4 km/h

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2 km/h

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8 km/h

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Solution

The correct option is A 3 km/h
$\frac{91}{S_{B} + S_{S}} + \frac{91}{S_{B} - S_{S}} = 20 \frac{91}{10 + S_{S}} + \frac{91}{10 - S_{S}} = 20 S_{S} = 3 k m / h r .$

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