For the first 5 s, the body traveled 20 m; for the next 80 m it moved at a speed of 10 m / s. What is the average speed?

Average speed is the entire path divided by the entire travel time, that is, your entire path is 20 meters + 80 meters = 100 meters, and all the time is 5 seconds + 8 seconds (time = path / speed) = 13 seconds.
From here it is already possible to find the average speed – this is 100 meters / 13 seconds of the way The answer is written as follows: the average speed is 100/13 m / s.