Posted by Lindsey please help on .
Based on data from the National Center for Health Statistics, N. Wetzel used the normal distribution to model the length of gestation for pregnant US woman ( Chance, Spring 2001). Gestation length has mean of 280 days with a standard deviation of 20 days.
a. Find the probability that gestation length is between 275.5 and 276.5 days. (This estimates the probability that a woman has her baby 4 days earlier than the due date)
b. Find probability that gestation length is between 258.5 and 259.5 days. (This estimates the probability that a woman has her baby 21 days earlier than the due date).
c. Find the probability that gestation length is between 254.5 and 255.5 days. (This estimates the probability that a woman has her baby 25 days earlier than the due date.
d. The Chance article referenced a newspaper story about three sisters who all gave birth on the same day (March 11, 1998). Karralee had her baby 4 days early: Marrianne had her baby 21 days early: and Jennifer had her baby 25 days early. Use the results, parts ac estimate the probability that all three woman have their babies 4, 21, 25 days early, respectively. Assume the births are independent events.

stats. 
PsyDAG,
ac. Z = (scoremean)/SD
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportions/probabilities related to the Z scores.
d. If the events are independent, the probability of both/all events occurring is determined by multiplying the probabilities of the individual events.