In a first order reaction, what fraction of the reactant will remain after 4 half-lives?

The answer is 1/16 because pretend you have a whole pie and every time you slice it, you cut it in halves...so therefore O is the whole pie, after the 1st half there's 1/2, after the second half life there is 1/4, after the third half life there is 1/8 and after the fourth half life you have 1/16 left. Hope I made it clear enough.