One car moving on a staright road covers one-third of the distance with 20 `(km)/(hr)` and the rest with 60 `(km)/(hr)`. The average speed is

Options

A. `40 km h^(-1)`
B. `80km h^(-1)`
C. `46(2)/(3)km h^(-1)`
D. `36km h^(-1)`

Answer

d

Explanation

Let the total distance be x. the car covers `(x)/(3)` in time `t_(1)`, with velocity `v_(1)=20` km/h
`therefore t_(1)=(x//3)/(20)=(x)/(60)`
Again it covers `(2x)/(3)` in time `t_(2)` with velocity `v_(2)=60` km/h
`therefore t_(2)=(2x)/(3xx60)=(x)/(90)`
Average speed `=("total distance coverd")/("total time")`
`=((x)/(3)+(2x)/(3))/((x)/(60)+(x)/(90))`
`=(60xx90)/((90+60))=36` km/h