Question

How long does it take for 87.5 % of each isotope(Krypton-73, Krypton-74, Krypton-76, and Krypton-81) to decay?

Answer 1

1st half-life: 100%/2 = 50%
2nd half-life: 50%/2 = 25% + 50% (from first half-life)
3rd half-life: 25%/2 = 12.5% + 75% (from first and second half-lives

I looked it up...