Use the Rule of 72 calculator. Enter 3% with an initial investment of $2000 in the first box. Enter 3% and the initial investment of $2500 in the second box. How much more will you make after 55 years by investing an extra $500 initially?
Couldn't you just, you know, "enter 3% with an initial investment of $2000" in the actual box? This is literally just plugging numbers into a screen and reading off the output.