The current of a stream runs at 1 km/h. A motorboat goes

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The current of a stream runs at 1 km/h. A motorboat goes 35 km upstream and back again at the starting point in 12 h. What is the speed of motorboat in still water ?

1. 7 km/h
2. 4 km/h
3. 6 km/h
4. 5 km/h

Correct Option: C

Let the speed of boat in still water be V km/h. Then,
35/(V - 1) + 35/(V + 1) = 12
⇒ 35(V + 1 + V - 1) = 12(V² - 1)
⇒ 6V² - 35V - 6 = 0
⇒ (V - 6)(6V + 1) = 0
⇒ V = 6 (V = - 1/6 is not possible)
So, speed of boat in still water is 6 km/h.