The accounting department at Weston Materials,Inc. a national manufacturere of unattached garages, reports that it takes two construction workers a mean of 32 hours and a standard deviation of 2 hours to erect the Red Barn Model. Assume the assembly times follows the normal distribution.

a) Deteremine the z values for 29 and 34 hours. What percent of the garages take between 32 hours and 34 hours to erect?
b) What percent of the garages take between 29 hours and 34 hours to erect?
c) What percent of the garages take 28.7 hours or less to erect?
d) Of the garages, 5 percent take how many hours or more to erect?


I have really tried this problem over and over, I can't seem to get pass letter A. This what I've come up with. Could someone please assist me with this?

I used the formula: z=x-the mean of the distribution/the standard deviation of the distribution

a) 29-32/2=1.5 34-32/2= 1


I'm not sure what I am to do next. I'm not sure if what I did thus far is correct. Help please.

I think you meant to say:

(29-32)/2 = -1.5 and (34-32)/2 = 1

So your z values are -1.5 and 1 and you've solved the first part of the question. I assume you know that you need to look up the z-values in the normal distribution chart to find out the probabilities for each.

Now think about what the second question is asking. What percentage of the garages take between 32 and 34 hours to erect? Well, if you know the mean is 32 then it should be obvious to you that 50% of the time a garage is erected in less than 32 hours and the other 50% of the time is takes more than 32 hours.

Now consider this equation: Pr(it takes less than 34 hours to erect a garage) - Pr(it takes less than 32 hours to erect a garage)

This equation is giving you the probability that a garage takes between 32 and 34 hours to erect.

Let me know if you still have questions!