## Elementary Statistics (12th Edition)

If $\hat{p}$ is known:$n=\frac{z^2_{\frac{\alpha}{2}}\cdot \hat{p}\cdot (1-\hat{p})}{E^2}.$ If $\hat{p}$ is unknown:$n=\frac{z^2_{\frac{\alpha}{2}}\cdot0.25}{E^2}.$ Here, $\hat{p}$ is unknown, hence $n=\frac{1.645^2\cdot0.25}{0.03^2}=752.$