The commute times during rush hour traffic on a local interstate have a mean of 25 minutes and a standard deviation of 5 minutes. Repeated studies of this section of interstate are considered normally distributed. What percent of commuters drive between 20 - 25 minutes to work daily?

Question

A) 16%
B) 32%
C) 34%
D) 50% <my choice

Answer 1

Disagree.

Z = (score-mean)/SD

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability of the Z score. Multiply by 100.

Answer 2

You should have learned that 68% fall between +1 and -1 deviations of the mean.
Since 20-25 is one standard deviation below the mean, without any calculation I know it is appr 34%

using my favourite stats site
http://davidmlane.com/normal.html
I got .3413 or 34.13%