Answer
752
Work Step by Step
$\alpha=1-0.9=0.1.$ Then, by using the table, the critical value: $z_{\alpha/2}=z_{0.05}=1.645.$
$\hat{p}$ is unknown, hence:$n=\frac{z^2_{\frac{\alpha}{2}}\cdot0.25}{E^2}=\frac{1.645^2\cdot0.25}{0.03^2}=752.$
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