How do you find the average rate of change of a function over an interval?

Average rate of change over an interval =

(difference in value of the function between the
beginning and end of the interval)
       divided by
(length of the interval).

Example:

If the function was 200 pounds at the beginning, and 98 pounds 3 hours later,
then the average rate of change is  (98 - 200) / 3 = -34 pounds per hour.