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The S&P 500, over its history, has returned north of 9% per year.

See my edit, I was talking about real returns including inflation.

The nominal return (9%) is:

40000 * (1.09 ^ 70) = $16,669,203.

The first calculation uses 7%. It is the real return. You'll actually have $16m, but the purchasing power would be $4.5m in 2021 dollars.