#### Answer

(a) 0
(b) no net change in the population during the 20 years

#### Work Step by Step

(a) We see from the graph that $P(40)=40$ and $P(2)=40$. We use this information to find the average rate of change of the population:
$\displaystyle \frac{P(40)-P(20)}{40-20}=\frac{40-40}{40-20}=\frac{0}{20}=0$
(b) Since the average rate of change was 0, the population did not change overall during this period.
In other words, the increases and decreases in the population cancelled each other out during the 20 years.