If we know the half life how could we determine what percentage of radioactive isotopes will be left after 4 half-lives?

"After 1 half-life there will be 50% of the original isotope, and 50% of the decay product.

After 2 half-lives there will be 25% of the original isotope, and 75% of the decay product.

After 3 half-lives there will be 12.5% of the original isotope, and 87.5% of the decay product.

After 4 half-lives there will be 6.25% of the original isotope, and 93.75% of the decay product." -Stephen Frantz, Former Director of the Reed College Nuclear Reactor

Explanation: