Joe is making $40K but, at retirement, in thirty years, without kids or a mortgage, he's sure he can live comfortably on $30K.
Assuming 3% inflation, $30K would be equivalent to 1.03^30 x $30K or $72,818 ... in thirty years, when he retires.
Using the notorious 4% Safe Withdrawal Rate, Joe would have to have a portfolio where $72,818 is 4%, namely 72,818/0.04 or a portfolio worth $1,820,450.
Joe figure's he'll try for a $1.8M portfolio.

QUESTION: If Joe's $40K salary keeps up with inflation, increasing 3% per year, what percentage of his salary should be invested, assuming an 8% annual return on investments, in order to achieve a $1,800,000 portfolio, in thirty years?
(A) 10% of his salary?
(B) 15% of his salary?
(C) 20% of his salary?
(D) none of the above

If'n y'all ain't got yer cal-cool-ator, y'all kin use this one.