After two half-lives, what fraction of a radioactive sample has decayed?1/2 2/3 3/4 5/6

A half life means that half the sample decays.

If the sample size is 1, then after one half life there would be x * 1/2 = 1/2 of the sample left.

For the 2nd half life, multiply 1/2 by 1/2 to get 1/4.

So after 2 half lives, there is 1/4 of the sample left, which means 3/4 of the sample decayed.

The answer is 3/4