Did I Get This – Using the Standard Deviation Rule
Length (in days) of human pregnancies is a normal random variable (X) with mean 266, standard deviation 16.
For the questions below, it would be useful to sketch this normal distribution yourself, marking its mean and the values that are 1, 2, and 3 standard deviations below and above the mean.
http://phhp-faculty-cantrell.sites.medinfo.ufl.edu/files/2013/02/qzDIG_08010.swf
This document is linked from Normal Random Variables.