Multiple choice

A boat takes 18 minutes and 30 minutes to cover a certain distance downstream and upstream respectively. If the speed of the boat in still water is 20 km/h, find the distance of one side.

  1. 15 km

  2. 8.5 km

  3. 7.5 km

  4. Cannot be determine

  5. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let distance be d and stream speed be c. Downstream: d = (20 + c) × (18/60) = (20 + c) × 0.3. Upstream: d = (20 - c) × (30/60) = (20 - c) × 0.5. Equating: 0.3(20 + c) = 0.5(20 - c). Solving: c = 5 km/h. Then d = (20 + 5) × 0.3 = 7.5 km.

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