Bigger animals tend to carry their young longer before birth. The length of horse pregnancies from conception to birth varies according to a roughly Normal distribution with mean 336 days and standard deviation 6 days. Let X = the length of a randomly selected horse pregnancy.

a. Write the event “pregnancy lasts between 325 and 345 days” in terms of X. Then find this probability.

b. Find the value of c such that P(X ≥ c) = 0.20.