A motorboat goes 10 km upstream and back in 55 minutes. Speed of stream is 2 km/h. Find speed of boat in still water.

Question

Accepted Answer

(C) 24 km/h. Explanation: Let speed = b. 10/(b-2) + 10/(b+2) = 55/60 = 11/12. Solving: 12×10×2b = 11(b²-4) → 240b = 11b²-44 → 11b²-240b-44=0 → b=22? Let's try b=22: 10/20+10/24 = 0.5+0.417=0.917≠55/60. Try b=24: 10/22+10/26 = 0.454+0.384=0.838≠. Answer is 22 km/h by standard formula.