Arithmetic

Time, Speed & Distance

Medium

A motor boat can travel at 10 km/hr in still water. It travelled 91 km downstream in a river and then returned to the starting point, taking altogether 20 hours. Find the rate of flow of the river?

5 km/hr

6 km/hr

3 km/hr

8 km/hr

✅ Correct Option: 3

Let the stream speed be $x$ . Then $\frac{91}{10+x} + \frac{91}{10-x} = 20$ , so $91 \times 20 = 20(100 - x^2)$ , giving $1820 = 2000 - 20x^2$ , hence $20x^2 = 180$ , $x^2 = 9$ and $x = 3$ km/hr.

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A motor boat can travel at 10 km/hr in still water. It travelled 91 km downstream in a river and then returned to the starting point, taking altogether 20 hours. Find the rate of flow of the river?

Solution

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