The average rate of change from x = 0 to x = 15 is about −4.667. how does the average rate of change from x = 0 to x = 20 compare to this number?

suppose that the height of the slide is 2 feet, when x = 20. what is the average rate of change over the entire slide?

First we calculate the ambio rate from x = 0 to x = 15:
 We have:
 R = ((80-10) / (0-15))
 R = -4.666666667
 We now calculate for the entire interval:
 from x = 0 to x = 20:
 R = ((80-2) / (0-20))
 R = -3.9
 
 What is the average rate of change over the entire slide?
 R = -3.9
 The average rate of change from x = 0 to x = 15 is about -4.667. How does the average rate change from x = 0 to x = 20 compare to this number?
 The percentage difference is:
 (-3.9 / -4.666666667) * (100) = 83.57142857%
 100-83.57142857 = 16.42857143%