Question

After two half lives, what fraction of a radioactive sample has decayed?

Answer 1

Well, `3/4` of the original isotope has decayed........

Explanation:

After `"1 half life"`, clearly HALF of the original isotope remains. And after TWO half-lives, a `"HALF + HALF of a HALF"`, i.e. `3/4` of the original mass has decayed.