the average speed of the car

Use the formula: distance = speed * time
Let time1 be the time used to travel to the distinaiton.
Let time2 be the time used to return to the original position.
Distance to the distination = 60*time1
Distance for the return trip = 20* time2
60*time1 = 20*time2 (because distance to the distination = distance for the return trip)
time2=3*time1
Total distance = distance to the distination + distance for the return trip
=60*time1+20* time2
=120*time1
Total time = time1 + time2
=time1+3*time1 (because time2=3*time1 as shown above)
=4*time1
Average apeed = total distance / total time
=120*time1 / (4*time1)
=30
The average speed of the car is 30 km/h

SET V=(2S)/(T+t)
WHEN 60=S/T , 20=S/t
THEN 60T=20t
3T=t
AND THEN V=(2S)/(3t+t)
=1/2(S/T)=1/2*60
=30 km/h

120/4=30km

let the distance be X km
then the car go to the destination in X/60hour
and it come back in X/20hour
the average speed of car:
2X/(X/60+X/20)
=2X/(4X/60)
=2X*60/4X
=30km/h

Suppose the length of the road is 60km
the car need 1 hour to go and 3 hours to return

so the car used 4 hours to travel 120km
so the average speed is 120/4 = 30km/h

(60+20)/2
=40km/h