Applying the formula isn't that hard, but this one could be hard to write about because it depends upon the amount of time the sum is compounding or the exponent in the equation.

^2 and ^3 are quite easy but when it comes to ^7/^9 there's no easy way to calculate mentally.

There is a method called the rule of 72. You can look it up but it's essentially: Years to Double = 72 / Annual Rate of Return