A launch travels at 15 km/h in still water. Its average speed for a trip upstream and back to the start is 14.4 km/h. What is the speed of the current?

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Accepted Answer

3 km/h. Round-trip average = 2(b + s)(b − s)/(2b) = (b² − s²)/b. So (225 − s²)/15 = 14.4 ⇒ s² = 9 ⇒ s = 3 km/h.

A launch travels at 15 km/h in still water. Its average speed for a trip upstream and back to the start is 14.4 km/h. What is the speed of the current?

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