How long will it take?

There's an old Abbot and Costello joke where Costello describes a problem he's having. I've always liked this joke because of what it's playing around with mathematically.

Costello tells Abbot he has a problem. That he likes this girl, but she's only 10 and he's 40, she's 1/4 his age. So of course he can't have anything to do with her yet.

But if he waits 5 years, she'll be 15 and he'll be 45, so she'll now be 1/3 his age. Still not good.

But if he waits another 15 years, she'll be 30 and he'll be 60, she'll be just 1/2 his age!

So, he wonders, how long does he have to wait before they're the same age?

:)