Can someone help me I really don't get this one

Question

The table shows the number of trolley buses in operation in the United States during certain years.

Describe the rates of change in the number of buses during the time period.

(Table)
Year 1980 1985 1990 1995 2000
Number 823 676 832 885 951
of Buses

Answer 1

The changes from one period to the next are

-147 156 53 66

Since each period is 5 years long, the average yearly change during each period is roughly

-30 31 10 12

Not sure what else they're after.