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:

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%

did anyone actually do this? like if anyone has submitted this i rly need .