Answer
$97$
Work Step by Step
$n=(\frac{z_{\alpha/2}\sigma}{E})^2$ (we have to round up)
$1-\alpha=0.95$, thus according to the table, $z_{\alpha/2}=z_{0.025}=1.96$.
$n=(\frac{1.96\cdot3000}{600})^2=96.04\approx97$
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