A car moves from A to B with a speed of 30 kmph and from B to A with a speed of 20 kmph. What is the average speed of the car ?

Options

A. 50 kmph
B. 25 kmph
C. 10 kmph
D. 24 kmph

Answer

D

Explanation

Let S be distance between A and B.
Let `t_(1)` be time taken by the car to move from A to B with speed `v_(1)` and `t_(2)` be time taken by the car to move from B to A with speed `v_(2)`. Then
`t_(1)=(S)/(v_(1))` and `t_(2)=(S)/(v_(2))`
Average speed of the car
`v_(av)=("Total distance travelled")/("Total time taken")=(2S)/(t_(1)+t_(2))`
`=(2S)/((S/(v_(1))+(S)/(v_(2))))=(2v_(1)v_(2))/(v_(1)+v_(2))`
Here, `v_(1)=30` kmph, `v_(2)=20` kmph
`:.v_(av)=(2xx30xx20)/(30+20)=24` kmph