Saving to retire is easy as 1-2-3, or a combo

— -- If you were good at word problems in high school, you know that if a Volkswagen Beetle left Chicago at 4:30 and traveled at the speed of light to Denver, it would still leak oil.

But here's a tougher one: How much money do you need if you retire at 62 and don't know how long you'll live or what your rate of return will be? Unlike previous generations, this pool of workers is projected to live nearly as long in retirement as they did in the workplace.

Give up? You're not alone. But it's a problem you're going to have to figure out when you retire. You can attack the problem in three ways, none of which gives an entirely satisfactory solution.

Method 1: The 4% solution

Let's say you have $1 million in an individual retirement account when you retire. And suppose you need $60,000 a year for living expenses, and your tax rate is 25%. You'll need to withdraw $80,000 your first year of retirement ($60,000 for you, $20,000 for the IRS). If you earn 8% a year on your investments, you can take out $80,000 a year forever and not touch your principal. Unfortunately, there are two problems with this approach:

•You can't earn a guaranteed 8% anywhere these days. An ultrasafe 30-year Treasury bill, for example, now yields just 4.25%.

•Inflation will slowly erode your $80,000. After 30 years of 3% inflation, $80,000 will be left with the buying power of only about $33,000. To combat inflation, you'll have to increase your withdrawals periodically.

The good news: If you have 30 years to invest, you can put some of your money into investments such as stocks and corporate bonds that should, over the long term, return more than 30-year Treasury bills.

Several academic studies have shown that if you want to take money out for 30 years, you should keep your money in a mix of stocks and bonds and take a low initial withdrawal — 4% to 5% of your savings. But that's only if you increase your withdrawals each year to account for inflation. You can take out more money if you forgo inflation adjustments for your withdrawals in years when the stock market drops sharply.