The first half of the journey, the car moved at a speed of 60 km / h, and the second half

The first half of the journey, the car moved at a speed of 60 km / h, and the second half – at a speed of 109 km / h. Find the average vehicle speed.

We denote the entire distance as S, then the time of passing the first section: (S / 2) / 60
the corresponding second: (S / 2) / 109. By definition of the average, the entire path must be divided by all the time, then we have:
v = S / (S / 120 + S / 109)
can be reduced by S
1 / (1/120 + 1/109) = 84.5