Multiple choice

A car driver covers a distance between two cities at a speed of 60 kmph and on the return his speed is 40 kmph. He goes again from the 1st to the 2nd city at twice the original speed and returns at half the original return speed. Find his average speed for the entire journey.

  1. 35

  2. 40

  3. 45

  4. 50
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Average speed = Total distance / Total time. Let distance between cities be D. Time 1 = D/60, Time 2 = D/40, Time 3 = D/120, Time 4 = D/20. Total time = D(1/60 + 1/40 + 1/120 + 1/20) = D(2/120 + 3/120 + 1/120 + 6/120) = 12D/120 = D/10. Total distance = 4D. Average speed = 4D / (D/10) = 40 kmph.

AI explanation

Let the one-way distance between the cities be D; applying the average speed formula for equal distances, 2D divided by the sum of the times for all four legs of the journey. The four times are D/60, D/40, D/120, and D/20, which sum to (2D + 3D + D + 6D) / 120 = 12D / 120 = D/10. The total distance traveled is 4D, so the average speed is 4D / (D/10) = 40 kmph. His average speed for the entire journey is 40.

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