Average Speed = $\frac{2 \cdot \text{Speed1} \cdot \text{Speed2}}{\text{Speed1} + \text{Speed2}}$

Speed1 = 40 km/hr (from A to B) Speed2 = 60 km/hr (from B to A)

Average Speed = $\frac{2 \cdot 40 \cdot 60}{40 + 60}$ = $\frac{4800}{100}$ = 48 km/hr