A boat goes 24 km upstream, halts there for 1 hour, and returns on a river flowing at 2 km/h. Its average speed for the whole outing, halt included, is 8 km/h. What is its speed in still water?

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Accepted Answer

10 km/h. Total time = 48 ÷ 8 = 6 h, so sailing time = 5 h. 24/(b − 2) + 24/(b + 2) = 5 ⇒ 48b = 5b² − 20 ⇒ 5b² − 48b − 20 = 0 ⇒ b = 10 km/h.

A boat goes 24 km upstream, halts there for 1 hour, and returns on a river flowing at 2 km/h. Its average speed for the whole outing, halt included, is 8 km/h. What is its speed in still water?

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