The first third of the way the car drove at a speed of 60 km / h, and the rest of the way

The first third of the way the car drove at a speed of 60 km / h, and the rest of the way at a speed of 80 km / h. Determine the average speed.

Average vehicle speed:
Vav. = S / t, where S is the common path (S = S1 + S2, where S1 = 1/3 S, S2 = 12/3 S), t is the travel time (t = t1 + t2).
S1 = V1 * t1, S2 = V2 * t2, where V1 = 60 km / h, V2 = 80 km / h.
Travel time when passing segments S1, S2:
t1 = S1 / V1 = (1 / 3S) / 60; t2 = S2 / V2 = (2 / 3S) / 80.
Let’s calculate the average speed:
Vav. = S / t = S / ((1 / 3S) / 60 + (2 / 3S) / 80) = S / (0.0056S + 0.0083S) = 1 / 0.0139 = 72 km / h.
Answer: The average speed is 72 km / h.