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A previous analysis of paper boxes showed that the standard deviation of their lengths is 13 millimeters. A packer wishes to find the 80% confidence interval for the average length of a box. How many boxes does he need to measure to be accurate within 3 millimeters?

a.14
b.19
c.51
d.31

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Let the number of boxes that have to be measured be N.

SD = 13 mm and ...

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Problem Solved.

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