Question

A car moves through 20km at a speed of 40km/h and the next 20km at a speed of 60km/h.calculate it's average speed

Answer 1

Average Speed = Total distance travelled / Total time taken 

Case 1 

distance = 20 km 
Speed = 40 km/h 
time = 20/40 = 1/2 h 

Case 1 

distance = 20 km 
speed = 60 km/h 
time = 20/60  = 1/3 h 

Speed (av) = 20 + 20 / (1/2 ) + (1/3) 
                  = 40 / 3 + 2 /6 = 40 x 6/5 = 240 /5 = 48 km/h 

Answer 2

Average speed = 2v1v2 / (v1 + v2)
= 2 × 40 × 60 / (40 + 60)
= 48 kmph

Average speed of car is 48 kmph