Multiple choice

A boat has a speed of $15$m/hr in still water. In a stream that has a current of $5$ m/hr it travels a certain distance downstream and returns. The ratio of the average speed for the round trip to the speed in still water is:

  1. $\dfrac{5}{4}$
  2. $\dfrac{1}{5}$
  3. $\dfrac{8}{9}$
  4. $\dfrac{7}{8}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let distance be d. Downstream speed = 15 + 5 = 20. Upstream speed = 15 - 5 = 10. Average speed = 2 * 20 * 10 / (20 + 10) = 400 / 30 = 40/3. Ratio to speed in still water (15) = (40/3) / 15 = 40/45 = 8/9.

AI explanation

The boat's downstream speed is 15 + 5 = 20 m/hr and its upstream speed is 15 - 5 = 10 m/hr. Using the average speed formula for a round trip, 2 times 20 times 10 divided by (20 + 10), the average speed is 400/30, or 40/3 m/hr. The ratio of this average speed to the speed in still water is (40/3) divided by 15, which simplifies to 8/9.

Keep practicing — related questions