Monthly Payments + Interest Problem

Hello!

I have a problem that I'm trying to figure out how to do the steps of. I have the solution, but I can't figure out how to get to it. The problem is as follows:

"Gina wants to establish a college fund for her newborn daughter that will have accumulated $120,000 at the end of 18 yr. If she can count on an interest rate of 6%, compounded monthly, how much should she deposit each month to accomplish this?"

The answer is $309.79, but I don't see how. Can someone please explain the steps to me? Thanks in advance.