Passengers board an elevator at the ground floor (i.e., the zeroth floor) and take it to the seventh floor, which is 84 feet above the ground floor. The elevator's position $s$ as a function of time $t$ (measured in seconds) is shown in Figure 14.
(a) What is the average velocity of the elevator from the time the elevator began moving until the time that it reached the seventh floor?
(b) What was the elevator's approximate velocity at time $t=20 ?$
(c) How many stops did the elevator make between the ground floor and the seventh floor (excluding the ground and seventh floors)? On which floors do you think the elevator stopped?