I'm assuming that the money is compounded annually.
P = principal interest
like morgaine says, do an example.
$100
after 1 year
$106.50
after 2 years
$113.4225
so on and so forth.
the actual formula can be found
Regular Compund Interest Formula
hint: if you put in P amount, and you wanna double it, the end product you want is 2P
...2P or NOT 2P