Answer
$423$
Work Step by Step
$n=(\frac{z_{\alpha/2}\sigma}{E})^2$ (we have to round up)
$1-\alpha=0.9$, thus according to the table, $z_{\alpha/2}=z_{0.05}=1.645$.
$n=(\frac{1.645\cdot125}{10})^2=422.82\approx423$
The Basic Practice of Statistics 7th Edition
by Moore, David S.; Notz, William I.; Fligner, Michael A.
Published by
W. H. Freeman
ISBN 10:
146414253X
ISBN 13:
978-1-46414-253-6
Chapter 18 - Inference in Practice - Apply Your Knowledge - Page 425: 18.13
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