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what is the probability that a randomly slected pregnancy lasts less than 172 days?

The mean being 176 days and the standard deviation being 12
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Assuming that pregnancy lengths are normally distributed (you did not specify - the answer will be different if not), let L be the random variable associated with an individual pregnancy length.
Probability(L < 172) = P((L-176)/12 < (172-176)/12) = P(Z < -1/3) = 0.3694