It is very easy to find the number of years needed to double your money: divide 72 by the interest rate you get and that´s all.
For instance, if my bank account pays me 6% for my 10.000 euros, I will need 12 years to get 20.000 euros (10.000 euros more): 72/6=12.
Of course wd can use this calculation to know the interest rate we need to double my money in a specific number of years.
If instead of making calculations to double the amount of money, we want to triple the money, we will use 114 instead of 72, if 4 times, the number will be 144.
