Physics - Kinematics Question with Solution | TestHub

Let the distance between A and B be $D$. The time taken to travel from A to B is $t_1=D/v_1$.

The time taken to travel from B to A is $t_2=D/v_2$.

total time taken is $t_1+t_2=D/v_1+D/v_2=D(\frac{1}{v_1}+\frac{1}{v_2})=D\frac{v_1+v_2}{v_1{v}_2}$.

The average speed is given by $\frac{\text{Total Distance}}{\text{Total Time}}=\frac{2D}{D\frac{v_1+v_2}{v_1{v}_2}}=\frac{2v_1{v}_2}{v_1+v_2}$.

Substituting the given values, average speed $=\frac{2\times 40\times 60}{40+60}=\frac{4800}{100}=48\text{km/h}$.