Taking the average of two average rates of change?

I am working on a math Jeopardy project for my Calc AB class and I wanted to write a question similar to the following one, but I need to know how to solve it first. Could anyone show me the work to get the answer of 13% per year?

__Here is the __**Question** out of my Calc book: The table shows the estimated percentage P of the population of Europe that use cell phones. (Midyear estimates are given).

Year: 1998 1999 2000 2001 2002 2003

P: 28 39 55 68 77 83

Estimate the instantaneous rate of growth in 2000 by taking the average of two average rates of change. What are its units?

So my attempt at this problem was taking the rate of change of 1999 and then 2001 and then taking the rate of change between those two to get the average rate of change of those average rates of change at 2000? ah help please. I was totally off.