Boat A (10 km/h in still water) and boat B (15 km/h in still water) each do the same out-and-back run on a river flowing at 5 km/h. B's total time is what fraction of A's?

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Accepted Answer

9/16. Round-trip time ∝ b/(b² − s²). A: 10/75 = 2/15. B: 15/200 = 3/40. Ratio = (3/40) ÷ (2/15) = 45/80 = 9/16. (2/3 is just 10/15, ignoring the current.)

Boat A (10 km/h in still water) and boat B (15 km/h in still water) each do the same out-and-back run on a river flowing at 5 km/h. B's total time is what fraction of A's?

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