Multiple choice

A boat P travels 40 km upstream from point A to B in 10 hours and from point B to A downstream in 5 hours. Another boat Q can travel from point A to a point C, 30 km upstream, in 5 hours. At 10 am, P starts from A and goes towards B. At the same time, Q starts from C and proceeds towards A. What is the speed of the boat Q in still water?

  1. 6 kmph

  2. 8 kmph

  3. 10 kmph

  4. 12 kmph
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Boat P's upstream speed is 4 kmph (40/10) and downstream speed is 8 kmph (40/5). The speed of boat P in still water is (8+4)/2 = 6 kmph, and the stream speed is (8-4)/2 = 2 kmph. Boat Q travels 30 km upstream in 5 hours, so its upstream speed is 6 kmph. Since Q's speed in still water minus the stream speed (2 kmph) equals 6 kmph, Q's speed in still water is 8 kmph.

AI explanation

Boat P travels 40 km upstream in 10 hours, giving an upstream speed of 4 kmph, and returns downstream in 5 hours, giving a downstream speed of 8 kmph. The speed of the stream is (8 - 4) / 2 = 2 kmph. Boat Q travels 30 km upstream in 5 hours, so its upstream speed is 6 kmph; adding the stream speed gives its still water speed as 6 + 2 = 8 kmph.

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