Want to work out compound interest rates in your head?
Use the Rule of 72
ln 2 is 69.3% – which is approximately equal to 70 which is close to 72 – which has many factors making it easy to do mental division.
So to work out compound interests rates’ doubling times divide 72 by the Interest rate.
At 6% $100 becomes $200 in 72/6 = 12 years