Question

a car travels 30km at a uniform speed of 40 km/h.the next 30 km is travelled with a speed of 20km/h.find its average speed.

Answer 1

Average Speed = Total Distance / Total Time
Speed = Distance / Time
 Time = Distance / Speed

 t1 = 30 / 40 = 0.75 hours
 t2 = 30 / 20 = 1.5 hours

 Average Speed = (30 + 30) / ( 1.5+0.75)
Average Speed = 60 / 2.25
Average Speed = 60 / 2.25
 Average Speed = 26.667 km/hr

or formula base 
average speed = (2 x v1 x v2) / (v1 + v2)
                         = (2 x 40 x 20) / (40 + 20)
                         = 1600/60 
                         = 26.667 km/hr


i hope it will help you 
regards