Salman went from Delhi to Jaipur with a speed of 60km/hr. and return back with…
2024
Salman went from Delhi to Jaipur with a speed of 60km/hr. and return back with the speed of 90km/hr. Find the average speed.
- A.
74
- B.
68
- C.
72
- D.
60
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Show answer & explanation
Correct answer: C
Key idea: For equal distances, the average speed is the harmonic mean of the two speeds: average speed = 2 × speed1 × speed2 ÷ (speed1 + speed2).
Let the one-way distance be d. Total distance = 2d.
Time for going = d ÷ 60. Time for return = d ÷ 90. Total time = d(1/60 + 1/90) = d × 5/180 = d/36.
Average speed = Total distance ÷ Total time = 2d ÷ (d/36) = 72 km/hr.
Therefore the average speed for the round trip is 72 km/hr.