Assume I am saving X dollars per year every year, and every year I earn 10 percent on what I saved the previous year. e.g. if I am saving 100 dollars every year I would see the trend: 100, 210, 331, 464.1, 610.51, etc....
This is very easy to calculate in excel, but what is the underlying formula? I tried to work it out and got X*(number of years + interest rate^(number of years)) which is wrong. Any suggestions?
Note, this isn't homework I'm just trying to find something that lets me do a back of the napkin calculation of compounding interest without having to open excel.