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