The ratio of the speed of a boat in still water to the speed of the stream is 5:2. It takes 4 hours more to travel 42 km upstream than to travel the same distance downstream. Find the speed of the boat in still water.

12 km/hr
10 km/hr
15 km/hr
20 km/hr

Correct answer: 10 km/hr

Solution

Let the speed of the boat in still water be 5x and the speed of the stream be 2x. Then upstream speed = 3x and downstream speed = 7x. Using the time difference, $42/3x - 42/7x = 4$, we get $x = 2$, so the boat speed is 10 km/hr.

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