Compound Interest and the Rule of 72

Compound interest is interest that is calculated not only based on the initial, or principal investment, but also upon the accumulated investment. This is in contrast of regular, or simple interest, which is only based on the initial investment.
The equaltion to calculate compound interest is as follows:

Where:
P = future value
C = initial deposit
r = interest rate (expressed as a fraction: eg. 0.06 for 6%)
n = # of times per year interest in compounded
t = number of years invested

P = C(1+ r/n)nt

I remeber doing someething simular if not the same in SAT Prep class a year or so ago. However, we never talked about the rule of 72. The rule of 72 estimates the amount of time it would take to double an investment.