The first half of the journey, the speed is 90 km / h, the second half, the speed is 60 km / h. Find the average vehicle speed.
Let S denote the length of the entire path that the car has traveled.
Let’s find out how long it took the car to travel all the way.
According to the condition of the problem, the car drove the first half of the way at a speed of 90 km / h, and the second half of the way at a speed of 60 km / h, therefore, the car drove the first half of the way in (S / 2) / 90 = S / 180 hours, the second half the car traveled for (S / 2) / 60 = S / 120 hours, and the car traveled all the way in S / 180 + S / 120.
Then the average vehicle speed is:
S / (S / 180 + S / 120) = S / (S / 72) = 72 km / h.
Answer: The average vehicle speed is 72 km / h.
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