$\begin{aligned} n &=\frac{\text { total time }}{\text { half- life period }}=\frac{90}{30}=3 \\ N &=N_{0} \times\left(\frac{1}{2}\right)^{n} \\ &=600 \times\left(\frac{1}{2}\right)^{3} \\ &=\frac{600}{8} \\ &=75 \text { atoms } \end{aligned}$