Saving enough for retirement is an enormous challenge for today's workers. Many people estimate that their retirement costs will amount to $1 million or more. Meanwhile, their annual income doesn't permit them to defer much money -- if any -- to their 401(k) or IRA.

The good news is that you can use investments that will hopefully grow in value over time. And you might be surprised by how much you could wind up with, even saving just $100 per month. But whether that monthly sum is enough for retirement depends on the following three factors.

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Source Fool.com