The number of crimes that occurred in a certain city per 1000 people had decreased from 45.4 in 1920 to 45.2 in 1930. Find the average rate of change in the number of crimes per 1000 people that occurred from 1920 to 1930.

solution
for this case we can use the following formula for the average rate:
[tex]\text{Avg}=\frac{End\text{ Value- Start Value}}{1930-1920}=\frac{45.2-45.4}{10}=-0.02[/tex]For this case the answer would be:
-0.02 crimes per 1000 people per year