The lengths of a particular animal's pregnancies are approximately normally distributed, with mean

=267

days and standard deviation

=8

days.(a) What proportion of pregnancies lasts more than

271

days?(b) What proportion of pregnancies lasts between

257

and

273

days?(c) What is the probability that a randomly selected pregnancy lasts no more than

263

days?(d) A "very preterm" baby is one whose gestation period is less than

247

days. Are very preterm babies unusual?