Consider the case where someone wants to retire with a total monthly benefit of $1,800 a month that will continue indefinitely.

Now, let's assume that Social Security will pay this individual $900 a month at age 65 and $1,200 a month at age 70.

We will assume the following:

1) The person is currently 24.

2) Money invested will earn 8% interest.

3) Money saved to draw interest from only earns 1.5% (because the money accumulated is now in a long-term savings account).

The question is how much money must be put aside a month for each scenario.