Given: Speed by car = 30 km/h Speed by walk = 10 km/h Formula used: If equal distance is travel at two different speed, then calculating average speed is as follows: Average speed = 2(Product of speeds)/(Sum of speeds) Calculation: Average speed =(2×30×10)/(30+10) ⇒600/40 ⇒ 15 km/h Alternative method Let the distance travel in each speed is 30 km. Average speed = Total distance travelled/Total time taken ⇒(30+30)/[(30/30)+(30/10)] ⇒60/4 ⇒ 15 km/h