Question 11 options: the lengths of human pregnancies are normally distributed with a mean of 268 days and a standard deviation of 15 days. what is the probability that a pregnancy lasts at least 300 days?

Think of (or draw) a standard normal curve with mean 268 days and std dev 15 days.

Calculate the z-score for 300 days:

       300-268          32
z = -------------- = --------- = 2.13
           15                15

Now find the area under the curve to the right of z = + 2.13.  Alternatively, find the area under the curve to the left of this z and subtract the result from 1.00.

I used my TI-83's DISTR function normalcdf( to calculate the area to the right of 300 days:

normalcdf(300,infinity, 268, 15) = 0.016, or about 0.02.

Only about 2% of pregnancies will last 300 days or longer.