Explanation:
the average change rate of a function f(x) in an interval [x1 .. x2] is
(f(x2) - f(x1)) / (x2 - x1)
in our case that is
(7000 - 2500) / (20 - 0) = 4500 / 20 = 225
the population changed in average by 225 people per year from 1900 to 1920.