Answer
90 years.
Work Step by Step
After 1 half-life, amount remaining= $28\,mg\times\frac{1}{2}=14\,mg$
After 2 half-lives, amount remaining= $14\,mg\times\frac{1}{2}=7\,mg$
After 3 half-lives, amount remaining =$7\,mg\times\frac{1}{2}=3.5\,mg$
3 half-lives are required for 28 mg to decay to 3.5 mg.
$t=3\times t_{1/2}=3\times30\,y=90\,y$
