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Sounds like your math maybe right. SPY average maybe 12% depending on start date.
But say 10 years or more ;
...
It IS right, easy to calculate with various free calculators online. Start with zero starting capital, annual "interest rate" 10%, then type in the monthly amount and play with the years. Yearly compounding, not monthly !
Peter Lynch was of course completely correct on the math portion.
In addition, 20 years can be too short for the S&P 500 as it has long stretches of sideways action.
Over 40 years however, you'll catch a big upswing in the US stock market for sure.
1964 - 1981, awful period for US Stocks in aggregate.
1982 - 1999, fantastic period.


