Question

The time required to cook a pizza at a neighborhood pizza joint is normally distributed with a mean of 12 minutes and a standard deviation of 2 minutes. find the time for each event. (round your answers to 2 decimal places.)
a. highest 5 percent min.
b. lowest 50 percent min.
c. middle 95 percent min. to min.
d. lowest 80 percent min.

Answer 1

Focus on "middle 95 percent min. to min" first.  The central 95% of the area under the std. normal curve represents -2 std. dev. up to 2 std. dev.  We need to convert std. dev. here into the actual x (minute) values, using the z-score formula:
         x - 12
z = --------------    Let this equal -2 first and find x; then let this equal 2, and find x
             2
                                        x - 12
x again:                  -2 = ----------  =>  -4 = x - 12  => 8 = x
                                           2

This means, when we're considering the middle 95% of cases, the minimum cooking time is 8 minutes.  The max would then be 12+4 = 16 minutes.

Hope this helps you get started.  Try solving the other 3 parts.  Let me know if you need further help with this.