A- what is the average rate of velocity of the ball in feet per second for
0 < t < 1.
b- what is the average rate of velocity of the ball in the air for 0 < t < 4

In displacement-time graph, Scale factors:
20 Feet & 0.5 second for each instant

1) Velocity = Final position - Initial position / Final time - Initial time
v = 100 - 60 / 1 - 0
v = 40 / 1
v = 40

In short, Your Answer would be 40 ft/sec

2) Velocity = 0 - 60 / 4 - 0
v = -60 / 4
v = -15

In short, Your Answer would be -15 ft/sec
[ Negative sign indicates that it is decreasing due to opposite direction ]

Hope this helps!