Answer
7.5 mg
Work Step by Step
Number of half-lives= $\frac{t}{t_{1/2}}=\frac{24\,h}{6.0\,h}=4.0$
As the sample undergoes 4 half-lives, the initial amount decreases by half 4 times.
Amount remaining=$120\,mg\times(\frac{1}{2})^{4}=\frac{120}{16}=7.5\,mg$
