The function given by $y=f(x)$ shows the value of $ 5000$ invested at $5 \%$ interest compounded continuously, $x$ years after the money was originally invested.
a. Find the average amount earned per year between the 5th year and 10 th year.
b. Find the average amount earned per year between the 20th year and the 25 th year.
c. Based on the answers from parts (a) and (b), does it appear that the rate at which annual income increases is increasing or decreasing with time?