Problem

Ex: The mean shelf life of a spice is 13.4 weeks and the standard deviation is 1.8 weeks.
(a) Would a shelf life of 20 weeks be unusual? Why?
(b) Find the z-score for a shelf life of 15.5 weeks.
(c) What percentage of shelf lives would be expected to be between 13.4 and 15.5 weeks?

Answer

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Answer

About 38.2\% of shelf lives would be expected to be between 13.4 and 15.5 weeks

Steps

Step 1 :Given: mean (μ) = 13.4 weeks, standard deviation (σ) = 1.8 weeks

Step 2 :Calculate the z-score for a shelf life of 20 weeks: z=xμσ

Step 3 :z=2013.41.8=3.67

Step 4 :A shelf life of 20 weeks is unusual because its z-score (3.67) is outside the typical range of z-scores (-2 to 2)

Step 5 :Calculate the z-score for a shelf life of 15.5 weeks: z=xμσ

Step 6 :z=15.513.41.8=1.17

Step 7 :Using a standard normal distribution table or calculator, find the area between z-scores of 0 and 1.17, which is approximately 38.2%

Step 8 :About 38.2\% of shelf lives would be expected to be between 13.4 and 15.5 weeks

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