A boat goes 27 km down the Krishna, which flows at 4 km/h, and then 27 km up the Tungabhadra, which flows at 2 km/h, taking 4.8 hours of sailing in all. What is its speed in still water?

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Accepted Answer

11 km/h. 27/(b + 4) + 27/(b − 2) = 4.8 ⇒ 27(2b + 2) = 4.8(b² + 2b − 8) ⇒ b² − 9.25b − 19.25 = 0 ⇒ b = 11 km/h. Check: 1.8 h + 3 h ✓.

A boat goes 27 km down the Krishna, which flows at 4 km/h, and then 27 km up the Tungabhadra, which flows at 2 km/h, taking 4.8 hours of sailing in all. What is its speed in still water?

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