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

Well, $\frac{3}{4}$ of the original isotope has decayed........
After $\text{1 half life}$, clearly HALF of the original isotope remains. And after TWO half-lives, a $\text{HALF + HALF of a HALF}$, i.e. $\frac{3}{4}$ of the original mass has decayed.