Answer
3 days.
Work Step by Step
$4.0\,\mu Ci=64\,\mu Ci\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}$
$64\,\mu Ci$ decays to $4.0\,\mu Ci$ in 4 half-lives.
$\implies 4\,\text{half-lives}=12\,\text{days}$
Or half-life $t_{1/2}=\frac{12\,days}{4}=3\,days$
