Two launches with the same still-water speed set off at the same moment from opposite ends of a stretch of river and pass each other. After passing, the downstream launch needs 4 more hours to reach its end and the upstream launch needs 9 more hours. If the current is 2 km/h, what is their speed in still water?

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Accepted Answer

10 km/h. For two movers starting together towards each other, speed ratio = √(t₂/t₁) = √(9/4) = 3/2. (b + 2)/(b − 2) = 3/2 ⇒ b = 10. Check: speeds 12 and 8 meet after 6 h; 8 × 6 = 12 × 4 and 12 × 6 = 8 × 9 ✓.

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