A study of long distance phone calls made the corporate offices of a company showed the calls follow a normal distribution. The mean length of time per call was 4.2min and the standard deviation was 0.60min.

a) What fraction of the calls last between 4.2 and 5 minutes:)
b) What fraction of the calls last more than 5 minutes:)
c) Construct confidence intervals at a 90% and 99% significance level:mad:
d) Comment on the magnitude of the interval’s width:eek:

1)How do we construct confidence intervals? I just don't get it...
2)What do I have to comment??

With a normal distribution, it's possible to figure out the fraction of the total that fall between limits. In fact, you can find tables that show you the percentage of the area that lies between fractions of the standard deviation. Since you know the mean and the standard deviation, you can use those tables (or the formula that is used to draw the curve) to answer your question:

Distribution Tables (http://www.statsoft.com/textbook/sttable.html)

http://www.math.umn.edu/~armstron/4707/Z-Values.pdf

Note that the mean was 4.2 minutes and the standard deviation was 0.6 minutes. 5 minutes is (5-4.2)/0.6 = 1.333 standard deviations from the mean. So, to answer a), all you have to do is look at the tables and find the fraction of area between the mean and 1.333 standard deviations from the mean.

Once you do that, the other answers are simply extensions of this method.

How to figure standard deviations