Final answer:
The velocity of the person walking from the 20m mark to the 100m mark in 10 seconds is 8 m/s.
Step-by-step explanation:
The velocity of an object is calculated by dividing the distance it travels by the time it takes. In this case, the person walks from the 20m mark to the 100m mark in 10 seconds, so the distance traveled is 100m - 20m = 80m. Therefore, the velocity is 80m divided by 10s, which equals 8 m/s.