Answer: -1.6
Step-by-step explanation:
For a graph of a function, the rate of change of function is the slope of the line.
The slope of a line passing from (a,b) and (c,d) is given by :-

In the given picture, we are given a graph having two points (1965,140) and (1990,100).
Now, the rate of change of the function is given by :-

Hence, the population was changing at the rate of -1.6 people per year.