The plane flies a distance s1 = 500 km at a constant speed v1 = 1000 km / h, then falls into the area of the headwind

The plane flies a distance s1 = 500 km at a constant speed v1 = 1000 km / h, then falls into the area of the headwind, and the plane’s speed decreases to v2 = 800 km / h. At this speed, it flies the next s2 = 1200 km. What is the average aircraft speed for the entire flight?

The solution of the problem:
T1 = 500/1000 = 0.5
t2 = 1200/800 = 1.5
vser. = (S1 + S2) / (t1 + t2) = (500 + 1200) / (1.5 + 0.5) = 850 km / h
Answer: the average speed of the aircraft for the entire flight was 850 km / h