I'm doing my pre-calculus homework and I cannot find an answer to this problem in my book or anywhere. I am give the following graph
The question is the following:
Estimate the average rate of change from $t = 1950$ to $t = 1960$, from $t = 1960$ to $t = 1970$, and from $t = 1970$ to $t = 1980$.
The issue is I don't know how you are supposed to calculate average rate of change if I can't find the $y$-value based on the graph. I know how to calculate the average rate of change if I know the points/pairs with $$\frac{y_2-y_1}{x_2-x_1}$$
This is all I am given, no other values or tables.
Given the graph above we can conclude the following (some values of $x$ don't have an easy to identify $y$ value on the graph because the interval of the y-axis is large).