Multiple choice

A ship sails 30 km of a river towards upstream in 6 hours. How long will it take to cover the same distance downstream? If the speed of the current is ¼ th of the speed of the boat in still water.

  1. 2

  2. 4.5

  3. 5

  4. 3.6

  5. 5.5
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let boat speed in still water = b. Current speed = b/4. Upstream speed = b - b/4 = 3b/4. Downstream speed = b + b/4 = 5b/4. Distance = 30 km, upstream time = 6 hours. So (3b/4) × 6 = 30, giving b = 20/3 km/h. Downstream time = 30/(5b/4) = 30 × 4/(5b) = 120/(5 × 20/3) = 120 × 3/100 = 3.6 hours. ✓

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