A bus moves from stop A to stop B with a speed of 40 km/hr and then from stop B to stop A with a speed of 50 km/hr. What is its average speed ?

$Average \ speed = \frac{Total \ distance }{ Total \ time}$

Let the distance from stop A to stop B is D. Therefore total distance traveled is 2D

Hence Average speed = $\frac{2D }{\frac{D}{40} + \frac{D}{50}}$

$= 2 \times 40 \times 50 \div (40 + 50 )$

=> Average speed = 44.4km/h.