A car travels half of the distance with constant velocity 40 km/h and another half with a constant velocity of 60 km/h along a straight line. The average velocity of the car is

Suppose total distance  is $2x$ $km$ then

time taken in traveling the distance  first $x$ will be $t_1=\frac{x}{40}$ 

and the time taken in traveling the distance  second $x$ will be $t_2=\frac{x}{60}$ 

now the  average velocity can be calculated as $V=\frac{\text{total distance}}{\text{total time}}=\frac{2x}{\frac{x}{40}+\frac{x}{60}}=\frac{2\times 40\times 60}{40+60}=48Km/h$