Multiple choice

A motor boat whose speed is 15 km/hr in still water goes 40 km downstream and comes back in a total of 6 hours. The speed of the stream is

  1. 9 km/hr

  2. 7 km/hr

  3. 5 km/hr

  4. 3 km/hr
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let stream speed be s. Downstream speed = 15 + s, upstream speed = 15 - s. Total time = 40 / (15 + s) + 40 / (15 - s) = 6. 40 * (15 - s + 15 + s) / (225 - s^2) = 6. 40 * 30 / (225 - s^2) = 6. 1200 / (225 - s^2) = 6. 200 = 225 - s^2. s^2 = 25, so s = 5.

AI explanation

Let the speed of the stream be x km/hr, making the downstream speed (15 + x) km/hr and the upstream speed (15 - x) km/hr. The total time to travel 40 km downstream and 40 km back is 6 hours, so 40/(15 + x) + 40/(15 - x) = 6. Finding a common denominator gives 1200 divided by (225 - x^2) equals 6, meaning 225 - x^2 equals 200. Solving for x gives x^2 = 25, so the speed of the stream is 5 km/hr.

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