Answer
$217$
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, $z_{\alpha/2}=z_{0.025}=1.96$.
$n=(\frac{1.96\cdot7.5}{1})^2=216.09\approx217$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.