Assume that a normal model with a mean 263 days and standard deviation can describe the duration of human pregnancies 12 days.
a. What percentage of pregnancies should last between 265 and 270 days?
b. At least how many days should the longest 30% of all pregnancies last?
C. Suppose a certain obstetrician is currently providing prenatal care to 30 pregnant women. What is the probability that the mean duration of these patients' pregnancies will be less than 261 days?