Multiple choice

A boat can cover 30 km going upstream in 20 minutes and 40 km going downstream in 24 minutes. How much time will the boat take to travel 84 km downstream if the speed of the stream is doubled?

  1. 48 minutes

  2. 46 minutes

  3. 36 minutes

  4. 24 minutes

  5. NA
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A Correct answer
Explanation

Upstream speed = 30 / (20/60) = 90 km/h. Downstream speed = 40 / (24/60) = 100 km/h. Let boat speed be u and stream speed be v. u - v = 90, u + v = 100. 2u = 190, u = 95, v = 5. New stream speed = 2 * 5 = 10. New downstream speed = u + 2v = 95 + 10 = 105 km/h. Time for 84 km = 84 / 105 hours = 0.8 hours = 48 minutes.

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