Answer to Question #151986 in Statistics and Probability for prp

Question #151986
If the mean of breaking strength of a copper wire is 575 pounds with a standard deviation of 8.3 pounds how large a sample must be used in order that there be one chance in 100 that the mean breaking strength of the sample is less than 572 pounds?
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Expert's answer
2020-12-21T17:02:45-0500

z=(x-mean)/σ/n=(572575)/8.3/n\sigma/\sqrt{n}=(572-575)/8.3/\sqrt{n} or |z|=3n/8.33\sqrt{n}/8.3

we need to find that value of x for which

0.01=area to the right at the variate z

that is area to the left=1-0.01=0.99=0.5+0.49

From the areas under the standard normal curve the corresponding value of z is 2.33. Hence

|z|=3n/8.33\sqrt{n}/8.3 gives 2.33=3n\sqrt{n} /8.3

3n=2.338.3=19.339,n=6.446,n=6.4462=41.55=42\sqrt{n}=2.33*8.3=19.339,\sqrt{n}=6.446,n=6.446^2=41.55=42



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