The first half of the journey, the car drove at a speed of 50 km / h on the rest of the way, it drove half the time

The first half of the journey, the car drove at a speed of 50 km / h on the rest of the way, it drove half the time at a speed of 40 km / h and half the time at a speed of 20 km / h Find the average value of the vehicle speed module.

Average travel speed:
Vav. = S / t, where S is the total distance traveled (S = S1 + S2 + S3, where S1 = 1/2 S, S2 + S3 = 1 / 2S), t is the total travel time (t = t1 + t2 + t3 , t2 = t3).
Since t2 = t3, the average speed in the second half of the journey: Vav2 = (40 + 20) / 2 = 30 km / h.
S1 = V1 * t1, where V1 = 50 km / h.
t1 = S1 / V1 = (1 / 2S) / 50.
Vav. = S / (t1 + t2 + t3) = S / (1 / 2S / 50 + 1 / 2S / 30) = S / (0.01S + 0.0167S) = 1 / 0.0267 = 37.45 km / h.
Answer: The average speed is 37.45 km / h.