further to my previous reply yes you can predict future numbers, but for a 40 year future prediction you are likely to get a very uncertain answer, eg if you make 12% on average now, but in the past it has varied from -6% to +30%, then you COULD make -4%,3% and 0% in the next 3 years

Or you could make 9%, 21%, 24% in the next 3 years. Starting with $1000, at the start of year 4 you could have $989 or $1635

I believe you will end up with an answer something like, for $1000 investment, in 40 years you will have between $15000 and $210,000, ie a mean of $112,500

The nature of random (normal) variation means that you don't NECESSARILY get one good year for every bad year you have....