A should round to the number 2.
The correct formula is 2P = PeYr ; so with P=1, A should equal 2 if you multiply by e raised to i times n.
The rule says that to find the number of years required to double your money at a given interest rate, you just divide the interest rate into 72. For example, if you want to know how long it will take to double your money at eight percent interest, divide 8 into 72 and get 9 years
See the following link
The Rule of 72 (with calculator) - Estimate Compound Interest