Let total distance covered in journey `= d`
Speed of car in first half covering distance `d/2 = v_1`
Time taken by the car during first half of the journey `(t_1)= (d/2)/(v_1) = d/(2v_1)`
Speed of car in second half covering distance `d/2 = v_2`
Time taken by the car during second half of the journey `(t_2)= (d/2)/(v_2) = d/(2v_2)`
`:.` Average speed ` = d/(t_1+t_2) = d/(d/(2v_1)+d/(2v_2))`
` =2/(1/v_1+1/v_2) = (2v_1v_2)/(v_1+v_2)`
So, average speed of the car is `(2v_1v_2)/(v_1+v_2)`.
