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. 40km/hr
B. 80km/hr
C. `46(2)/(3) km//hr`
D. 36 km/hr

Answer

D

Explanation

`t_(1)=((d)/(3))/(20)=(d)/(60)` and `t_(2)=((2d)/(3))/(60)=(d)/(20)`
total time `t=t_(1)+t_(2)`
average speed `=(d)/(t)=(d)/((d)/(60)+(d)/(90))=36 km//hr`.
`=(60xx90)/(150)=36 km//hr`