Question

The average speed of a car running at the rate of 15 km/hour during the first 30 km and at 20 km/hour during the second 30 km and at 25 km/hour during the third 30 km, will be

• A. 19.15 km/hour
• B. 20 km/hour
• C. 19.65 km/hour
• D. 19.75 km/hour.

Answer 1

Answer: (A)

19.15 km/hour

Answer 2

Here, the required average speed is equal to the harmonic mean of the three speeds $= \frac{3}{\frac{1}{15} + \frac{1}{20}+ \frac{1}{25}}$ km/hour $= \frac{3\times300}{20+15+12} = \frac{900}{47}$ km/hour = 19.15 km/hour.