A car is moving on a straight road covers one third of the total distance at 20km/h, and the rest at 60km/h. What is its average speed?

To solve such sums let us assume some quantities first .

 Let the total distance covered be X km

It covers X/3 distance with speed 20km/hr. 

And rest 2X/3 distance with speed 60km/hr . 

As we have speed & distance we can find time taken.

Time taken for X/3km is , T1= (X/3)/20 = X/60 hr and

 Time taken for 2X/3km is , T2= (2X/3)/60 = X/90 hr 

Total time is T = X/60 + X/90 = X/36 hr

So average speed would be,

total distance/ total time

= X / (X/36)= 36 km/hr