On a calm day the river flows at 3 km/h; after heavy rain it flows at 5 km/h. A launch (15 km/h in still water) does the same out-and-back run on both days. By what percentage does the total time increase after the rain?

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Accepted Answer

8%. Round-trip time ∝ b/(b² − s²). Ratio = (225 − 9)/(225 − 25) = 216/200 = 1.08 ⇒ 8% longer.

On a calm day the river flows at 3 km/h; after heavy rain it flows at 5 km/h. A launch (15 km/h in still water) does the same out-and-back run on both days. By what percentage does the total time increase after the rain?

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