A car covers the first half of the distance between the two places at $40 {\mathrm{kmh}}^{-1}$. and another half at $60 {\mathrm{kmh}}^{-1}$. The average speed of the car is 

As from the question, Average speed of the car, vav                                         = Total distance travelled by the body (d) Total time taken t1+t2      …(i)So, now time taken in 1st half of the distance,                          t1= Time = Distance  Velocity =d2×40⇒                    t1 = d80                 (∵ Distance =d2, velocity =40kmh-1 ) Time taken for 2nd half, t2=d2× Velocity So,                                 t2=d2×60=d120                                                 (∵ distance =d2, velocity =60kmh-1 ) Now, average velocity, vav=dd80+d120              [From Eq. (i})                                          =d×80×120200d=48kmh-1