The runner ran for 4s at an average speed of 10m / s and 5s at a speed of 12m / s. At what average speed did he run the entire distance?

According to the conditions of the problem:
t1 = 4 seconds, v1 = 10 m / s – time and speed on the first section of the path;
t2 = 5 seconds, v2 = 12 m / s – time and speed on the second leg.
It is required to find Vcr. – average speed along the entire route.
Average speed is defined as the ratio of the total distance traveled to the total time, that is,
Vav = S / t.
We find the full path from the formula: S = S1 + S2 = v1 * t1 + v2 * t2 = 4 * 10 + 5 * 12 = 100 meters.
Total time: t = t1 + t2 = 4 + 5 = 9 seconds.
Hence, Vav = 100/9 = 11.1 m / s.
Answer: the average speed over the entire distance is 11.1 m / s.