A person travels from X to Y at a speed of 30 km/hr and returns from Y to X at a speed 50% more. What is the average speed for the whole journey?

A35 km/hr

B37.5 km/hr

C36 km/hr ✓ Correct

D40 km/hr

Correct answer: (C) 36 km/hr

Explanation

The average speed is 36 km/hr.

Return speed is 50% more than 30, so it is 45 km/hr.

For equal distances, average speed = 2ab/(a + b).

= 2 x 30 x 45 / (30 + 45).

= 2700 / 75 = 36.

This is the harmonic mean of the two speeds.

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