In: Statistics and Probability
The accounting department at Weston Materials Inc., a national manufacturer of unattached garages, reports that it takes two construction workers a mean of 33 hours and a standard deviation of 5 hours to erect the Red Barn model. Assume the assembly times follow the normal distribution. Refer to the table in Appendix B.1.
a-1. Determine the z-values for 30 and 37 hours. (Negative answers should be indicated by a minus sign. Round the final answers to 1 decimal place.)
30 hours corresponds to z =
37 hours corresponds to z =
a-2. What percentage of the garages take between 33 hours and 37 hours to erect? (Round the final answer to 2 decimal places.)
Percentage %
b. What percentage of the garages take between 30 hours and 37 hours to erect? (Round the final answer to 2 decimal places.)
Percentage %
c. What percentage of the garages take 29.4 hours or less to erect? (Round the final answer to 2 decimal places.)
Percentage %
d. Of the garages, 2% take how many hours or more to erect? (Round the final answer to 1 decimal place.)
Hours
Mean = 33 hours
Standard deviation = 5 hours
a-1. z = (X - mean)/standard deviation
30 hours corresponds to z = (30 - 33)/5
= -0.6
37 hours corresponds to z = (37 - 33)/5
= 0.8
a-2. P(X < A) = P(Z < (A - mean)/standard deviation)
P(between 33 hours and 37 hours) = P(33 < X < 37)
= P(X < 37) - P(X < 33)
= P(Z < (37 - 33)/5) - P(Z < (33 - 33)/5)
= P(Z < 0.8) - P(Z < 0)
= 0.7881 - 0.5
= 0.2881
= 28.81%
b. P(between 30 hours and 37 hours) = P(33 < X < 37)
= P(X < 37) - P(X < 30)
= P(Z < (37 - 33)/5) - P(Z < (30 - 33)/5)
= P(Z < 0.8) - P(Z < -0.6)
= 0.7881 - 0.2743
= 0.5138
= 51.38%
c) P(29.4 hours or less) = P(X < 29.4)
= P(Z < (29.4 - 33)/5)
= P(Z < -0.72)
= 0.2358
= 23.58%
d) Let H be the number of hours by 2% of garages
P(X > H) = 0.02
P(X < H) = 1 - 0.02 = 0.98
P(Z < (H - 33)/5) = 0.98
Take the z value corresponding to 0.9800 from standard normal distribution table
(H - 33)/5 = 2.05
H = 43.25 hours