The current of a stream runs at 1 km/hr. A motor boat goes 35 km upstream and back again to the starting point in 12 hours. The speed of motor boat in still water is?

6 km/hr
7 km/hr
8.5 km.hr
8 km/hr

Correct Answer: 6 km/hr

Explanation:

Let the speed in still water be x km/hr
∵ 35/(x - 1) + 35/(x + 1) = 12
⇒ 35(2x) = 12(x² - 1)
⇒ 12x² - 70x - 12 = 0
⇒ 12x² - 72x + 2x - 12 = 0
⇒ 12x(x - 6) + 2 (x - 6) = 0
⇒ (x - 6) (12x + 2) = 0
∴ x = 6 km/hr

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