I'm having trouble figuring out how to use Goal Seek to solve the number of periods (NPER) required.

The Data I have:
PV (Present Value)
FV (Future Value)
Rate (Interest)

and the scenarios I need this for are lump sum amounts amount of money, so it is one initial deposit and no payments (PMT = 0).

Again, I need to calculate NPER (The number of periods) for the PV to increase to FV at the Rate.

There are guides that have almost exactly what I need. Two easy examples here:
http://spreadsheets.about.com/od/adv...2goal_seek.htm
http://hubpages.com/hub/Calculating-...sing-Goal-Seek

I just can't wrap my brain around how to get this to calculate NPER instead of FV.

I can't figure out what value to change to get the number of periods. It has to be a formula, but which? And to what number would I be changing it to?

Thanks in advance for any help!
Going crazy being unable to figure this out on my own