Multiple choice

A boat can travel 15 km upstream in 5 hours. Find the time taken by the boat to cover the same distance downstream when the speed of the current is 1/4th the speed of the boat in still water.

  1. 6 hours

  2. 5 hours

  3. 3 hours

  4. 2 hours

  5. 4 hours
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Upstream speed = 15 km / 5 hours = 3 km/hr. Let boat speed be B and current speed be C. B - C = 3. Given C = 1/4 B, so B - 0.25B = 3, which means 0.75B = 3, so B = 4 km/hr and C = 1 km/hr. Downstream speed = B + C = 4 + 1 = 5 km/hr. Time = 15 km / 5 km/hr = 3 hours.

AI explanation

The upstream speed is 15 km divided by 5 hours, which gives 3 km/hr; since this equals the boat speed minus the stream speed, and the stream speed is one-fourth of the boat speed, we write B - B/4 = 3. Solving gives the boat speed as 4 km/hr and the stream speed as 1 km/hr, so the downstream speed is 5 km/hr. The required time is 15 km divided by 5 km/hr, which is 3 hours.

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