Letu2019s have some fun with numbers.2045 is in approximately 26 years, so we are looking at the possible 26-yr total returns. This includes all dividends and that dividends are immediately reinvested.So to get a reasonable estimate, we will need to look at historical returns for the S&P 500. I will be using annual total returns going back to 1988.Source:S&P 500 Annual Total Return (Yearly)This gives a data set of 31 annual returns.If you look at the average return, you get 11.62%Using that as our growth rate, we can project our growth factor.[math] g = (1+.1162)^{26} = 17.45 [/math]Multiplying by $20M gives about $349M.Not bad, but that is probably unlikely. Lets look at giving a more accurate projection in terms of a 95% confidence interval.Going back to the data set. The standard deviation was about 17.2%That is some significant variance, the range of returns was from -37% to 37.6%So to better eour forecast, lets get a better eof the true average return for the S&P 500.In statistics, this is known as getting a confidence interval for the true mean using standard error (SE).[math] SE = frac{s}{sqrt{n}} = frac{.172}{31} = .0309 [/math]Our efor the true average return is[math] x pm 2*SE = .1162 pm .0618 = (.0544, .178) [/math]Using these for our growth ratesu2026[math] g_{min} = (1+.0544)^{26} = 3.96 [/math][math] g_{max} = (1+.178)^{26} = 70.76 [/math]Multiplying these by $20M gives a range from $79M to $1.4BThis means you have a 95% chance of growing your $20M to somewhere within that range by 2045.In other words, itu2019s probably a good idea to invest it.