Question

The length of human pregnancies from conception to birth varies according to a distribution that is approximately normal with mean 266 days and standard deviation 16 days.

How long do the longest 20% of pregnancies last?

Answer 1

using a standard normal table, obtain the z-value with 80% to left/ 20% to right
http://en.wikipedia.org/wiki/Z_table#Cumulative_table

use "z-score" formula to obtain num of days(N)
\[Z = \frac{N-mean}{std dev}\]
\[0.84 = \frac{N-266}{16}\]
\[N = 279.44\]
-The longest 20% last more than 279 days