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A boat covers 8 km in still water and returns the same distance, taking 2 hours for the round trip in a calm lake. If the speed of the water current is 4 km h⁻¹, how long will it take to go 8 km upstream and then come back the same distance downstream?

1. 160 minutes
2. 80 minutes
3. 100 minutes
4. 120 minutes

Correct answer: 160 minutes

Solution

In still water 8 km each way in 2 h gives boat speed 16/2 = 8 km/h. With current 4 km/h: upstream speed 4 km/h takes 8/4 = 2 h; downstream speed 12 km/h takes 8/12 = 2/3 h. Total = 2 + 2/3 h = 8/3 h = 160 minutes.

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