Question

A pilot sample of 50 measurements of the breaking strength of cotton threads gave S_(pr) = 18 grams. Determine the sample size needed to obtain a 90% Confidence Interval of length 4. I tried using the sample size formula but that did not gave me the right answer, according to the book the answer is n = 220.
Figured out the answer:

n = (1.645(18/2))^2 = 219.188 = 220

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