The first half of its journey, the car moved at a speed of 90 km / h

The first half of its journey, the car moved at a speed of 90 km / h, and the second half of the journey – at a speed of 50 km / h. Determine the average speed of the vehicle.

v1 = 90 km / h – vehicle speed on the first half of the journey;

v2 = 50 km / h – vehicle speed on the second half of the journey.

It is required to find vav (km / h) – the average speed of the vehicle.

Let S be the entire distance traveled, and t – all the time spent moving. Then:

vav = S / t = S / (t1 + t2) = S / (S / (2 * v1) + S / (2 * v2)) =

= S / (S * (v1 + v2) / (2 * v1 * v2)) = 2 * v1 * v2 / (v1 + v2) =

= 2 * 90 * 50 / (90 + 50) = 9000/140 = 64.3 km / h.

Answer: The average vehicle speed is 64.3 km / h.