On a horizontal section of the road, the car moved at 36 km / h for 20 minutes and then

On a horizontal section of the road, the car moved at 36 km / h for 20 minutes and then drove downhill at 72 km / h in 10 minutes. Determine the average speed of the car along the way

V1 = 36 km / h = 10 m / s
V2 = 72 km / h = 20 m / s
t1 = 20 min = 1200 s.
t2 = 10 min = 600 s.
Vav-?
In order to find the average speed Vav, it is necessary to divide the entire path S by the time t, during which this path was covered Vav = S / t. t = t1 + t2; S = S1 + S2; S1 = V1 * t1; S2 = V2 * t2; Vav = (V1 * t1 + V2 * t2) / (t1 + t2); Vav = (10 m / s * 1200s + 20m / s * 600s) / (1200s + 600s) = 13.3 m / s
Answer: Vav = 13.3 m / s