If the interest earned by a cd is compounded quarterly which rule is most accurate when calculating how long it will take the money invested in the cd to double

If the interest earned by a cd is compounded quarterly and the money invested is doubled after a certain time. So, we are given an interest rate, an initial amount and a final amount. 

First, convert the interest rate to per yer compounded yearly. Next, use this formula to solve for the number of years to double the amount:

F = P (1+i)^n