Answer
28
Work Step by Step
Given $E=25, c=0.9, \sigma=80$
we have $\alpha/2=0.05, Z_{\alpha/2}=1.65$
then $n=(\frac{Z_{\alpha/2}\cdot\sigma}{E})^2=(\frac{1.65\times80}{25})^2\approx28$
The sample size should be at least 28.
Elementary Statistics: A Step-by-Step Approach with Formula Card 9th Edition
by Bluman, Allan
Published by
McGraw-Hill Education
ISBN 10:
0078136334
ISBN 13:
978-0-07813-633-7
Chapter 7 - Confidence Intervals and Sample - Review Exercises - Section 7-1 - Page 407: 3
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