The lengths of a particular animal's pregnancies are approximately normally distributed, with mean days and standard deviation 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?