Pregnancy Stats Questions (don't know how to get book's ans)


New member
May 29, 2008
Assume that the duration of human pregnancies can be described by a Normal model with mean 266 days and standard deviation 16 days.

A) What percentage of pregnancies should last between 270 and 280 days?

B) At least how many days should the longest 25% of all pregnancies last?

C) Suppose a certain obstetrician is currently providing prenatal care to 60 pregnant women. Let y (with line over it) represent the mean length of their pregnancies. According to the Central Limit Theorem, what's the distribution of this sample mean, y (with line over it)? Specify the model, mean, and standard deviation?

D) What's the probability that the mean duration of these patient's pregnancies will be less than 260 days?

The answers are:
A) 21.1%

B) 276.8 days or more

C) N(266, 2.07)

D) 0.002

I was hoping somebody can help me figure out HOW the book got to these answers. I am completely stumped!

Help please! Thanks


Full Member
Dec 14, 2005
Start by computing the appropriate z-scores and refer to your standard normal distribution table.