# Pregnancies vary acc to a roughly normal distrib w/mean 336 days/standard dev-3 days. Use the 68-95-99.7 rule:What % are 330-342 days? between 333-336 days?

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### 1 Answer

Since we have a normal distribution, roughly 68% of the data will lie within 1 standard deviation of the mean, 95% of the data will lie within 2 standard deviations of the mean, and 99.7% of the data will lie within 3 standard deviations of the mean.

With the given numbers: 68% of the data will lie within 3 days (the standard deviation) of the mean (336 days) so 68% of the data will fall from 333 to 339 days.

95% of the data will lie within 2 standard deviations of the mean -- thus 95% of births will take between 330 and 342 days.

99.7% of the data lies within 3 standard deviations of the mean -- then 99.7% of births take between 327 and 345 days.

**Thus, 95% of the data will lie between 330 and 342 days.**

For 333-336, we notice that 68% lies between 333 and 339 days. Half of these will lie between 333 and 336 days, so **34% lie between 333 and 336 days.**