The lengths of a particular animal's pregnancies are approximately normally distributed, with mean μ days and standard deviation σ days.
(a) What proportion of pregnancies lasts more than 268 days?
(b) What proportion of pregnancies lasts between 253 and 262 days?
(c) What is the probability that a randomly selected pregnancy lasts no more than 247 days?
(d) A "very preterm" baby is one whose gestation period is less than 241 days. Are very preterm babies unusual?