Multiple choice

It takes a total of 15 hours for a boat in still water to reach its destination and return to its starting place. The same journey requires 16 hours if the river flows. The difference between the speed of the boat and the river is 15 km/h. Find the speed of river flow.

  1. 10 km/h

  2. 6 km/h

  3. 4 km/h

  4. 5 km/h
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let boat speed = b and river speed = r. Given b - r = 15 km/h. In still water: 2D/b = 15 hours, so D = 7.5b. In flowing river: 2D/(b+r) + 2D/(b-r) = 16 hours. Substitute D = 7.5b: 15b/(b+r) + 15b/(b-r) = 16. Solving with b - r = 15 gives r = 5 km/h. Options A, B, C don't satisfy the time equation.

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