The lengths of a particularâ animal's pregnancies are approximately normallyâ distributed, with mean muμequals=270270 days and standard deviation sigmaÏequals=2020 days. â(a) what proportion of pregnancies lasts more than 275275 âdays? â(b) what proportion of pregnancies lasts between 255255 and 285285 âdays? â(c) what is the probability that a randomly selected pregnancy lasts no more than 230230 âdays? â(d) aâ "very preterm" baby is one whose gestation period is less than 240240 days. are very preterm babiesâ unusual?