A car moving on a straight road covers one third of a certain distance with 20 Km/h and the rest with 60 Km/h. The average speed is :

Given,

Velocity for one third of distance $V_1=20km/h$

Velocity for two third of distance $V_2=60km/h$

 

Time take to cover one third of distance $t_1=\frac{d/3}{V_1}=\frac{d/3}{20}$

Time taken to cover two third of distance $t_2=\frac{2d/3}{V_2}=\frac{2d/3}{60}$

$\text{Average velocity =}\frac{\text{Total}\text{distance}}{\text{total}\text{time}}$

 $V_{av}=\frac{d}{t_1+t_2}=\frac{d}{\frac{d/3}{20}+\frac{2d/3}{60}}=36km/h$