The car moved from point A to point B at a speed of 40 km / h, and moved back 30 km / h.

The car moved from point A to point B at a speed of 40 km / h, and moved back 30 km / h. Determine the average speed for the flight.

Let’s designate the distance AB as S, then the full path (there and back) is equal to 2S.
Average speed is equal to the ratio of the entire path to the entire time.
Let’s find the time at each site (at the site there, and at the site back):
t1 = S / 40
t2 = S / 30
Total time T = S / 40 + S / 30 = 7S / 120
Find the average speed:
Vav = 2S / (7S / 120) = 2S * 120 / 7S = 240/7 = 31.4 km / h