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Posted by Jane on Sunday, February 10, 2013 at 4:32pm.

The lengths of pregnancies are normally distributed with a mean of 273 days and a standard deviation of 20 days. If 64 women are randomly selected, find the probability that they have a mean pregnancy between 273 days and 275 days.

Use z-scores: z = (275 - 273)/(20/√64) = ? Once you calculate the z-score, check a z-table from mean to that z for your probability.

2/2.5 = 0.8 z=0.7881 1-0.7881 = 0.2119

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