The current of a stream runs at the rate of 4 km/hr. A boat goes 6 km and back to the starting point in 2 hours. The speed of the boat in still water is?

6 km/hr
7.5 km/hr
8 km/hr
6.8 km/hr

Correct Answer: 8 km/hr

Explanation:

Let the speed in still water be x km/hr
Then, 6/x + 4 + 6/x - 4 = 2
⇒ 6[ x - 4 + x + 4] = 2(x² - 16)
⇒ x² - 6x - 16 = 0
⇒ (x - 8) (x + 2) = 0
∴ x = 8 km/hr

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