A boat's speed in still water is 5km/h. While river is flowing

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A boat's speed in still water is 5km/h. While river is flowing with a speed of 2km/h and time taken to cover a certain distance upstream is 2 h more than time taken to cover the same distance downstream. Find the distance ?

1. 10.5 km
2. 11 km
3. 10.9 km
4. 15 km

Correct Option: A

Let the distance be D km.
Speed download = (5 +2) = 7 km/h
and speed upstream = (5 -2) = 3 km/h
According to the question,
D/3 - D/7 = 2
⇒ 7D - 3D = 21 x 2
∴ D = (21 x 2)/4 = 10.5 km