 a lump sum invested in a good mutual fund or series of mutual funds, if it averages 10% or more, will double every seven years. You're 37, you said? Yep, you and I. So let's pretend we put 400 in investments. When you're 44, you'll have 700. When you're 51, you'll have 1.4. When you're 58, you'll have 2.8.