A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is

Use average speed = total distance ÷ total time [1]. Let total distance = D, so each one-third segment = D/3. Times for the three segments are t1 = (D/3)/10 = D/30, t2 = D/60, and t3 = D/180. Total time T = D(1/30 + 1/60 + 1/180) = D(10/180) = D/18. Therefore average speed = D / (D/18) = 18 km/hr. This result is the harmonic-mean outcome for equal-distance segments and agrees with worked solutions for this three-segment case . Hence option 1 (18 km/hr) is correct.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > 8.3 Speed > p. 113