Answer
48, which is practical.
Work Step by Step
To be 95% confident that is within 20% of $\sigma$ by using the table, the corresponding sample size is: 48, which is quite small and hence practical.
Essentials of Statistics (5th Edition)
by Triola, Mario F.
Published by
Pearson
ISBN 10:
0-32192-459-2
ISBN 13:
978-0-32192-459-9
Chapter 7 - Estimates and Sample Sizes - 7-4 Estimating a Population Standard Deviation or Variance - Page 365: 20
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