How much of a 1.00 mg sample of iodine remains after 3 half-lives?

0.125 mg.

The mass of a radioactive species decrease by by the end of each half-life.

For this 1.00 mg sample of unknown half-life:

Mass of the sample at the end of the n-th half-life;

.

As a result,

of the sample remains after three half-lives.