In: Statistics and Probability
A factory produces plate glass with a mean thickness of 4mm and a standard deviation of 1.1mm. A simple random sample of 100 sheets of glass is to be measured, and the mean thickness of the 100 sheets is to be computed.
What is the probability that the average thickness of the 100 sheets is less than 3.91 mm?
Solution:- Given that mean = 4, sd = 1.1, n = 100
=> the probability that the average thickness of the 100 sheets is less than 3.91 mm is 0.2061
P(X < 3.91) = P((X-mean)/(sd/sqrt(n)) <
(3.91-4)/(1.1/sqrt(100)))
= P(Z < -0.8182)
= 1 - P(Z < 0.8182)
= 1 - 0.7939
= 0.2061
Now we can find P(Z < 0.8182) by using the standard normal
table
Z | 0.00 | 0.01 | 0.02 | 0.03 | 0.04 | 0.05 | 0.06 | 0.07 | 0.08 | 0.09 |
0.0 | 0.5 | 0.504 | 0.508 | 0.512 | 0.516 | 0.5199 | 0.5239 | 0.5279 | 0.5319 | 0.5359 |
0.1 | 0.5398 | 0.5438 | 0.5478 | 0.5517 | 0.5557 | 0.5596 | 0.5636 | 0.5675 | 0.5714 | 0.5753 |
0.2 | 0.5793 | 0.5832 | 0.5871 | 0.591 | 0.5948 | 0.5987 | 0.6026 | 0.6064 | 0.6103 | 0.6141 |
0.3 | 0.6179 | 0.6217 | 0.6255 | 0.6293 | 0.6331 | 0.6368 | 0.6406 | 0.6443 | 0.648 | 0.6517 |
0.4 | 0.6554 | 0.6591 | 0.6628 | 0.6664 | 0.67 | 0.6736 | 0.6772 | 0.6808 | 0.6844 | 0.6879 |
0.5 | 0.6915 | 0.695 | 0.6985 | 0.7019 | 0.7054 | 0.7088 | 0.7123 | 0.7157 | 0.719 | 0.7224 |
0.6 | 0.7257 | 0.7291 | 0.7324 | 0.7357 | 0.7389 | 0.7422 | 0.7454 | 0.7486 | 0.7517 | 0.7549 |
0.7 | 0.758 | 0.7611 | 0.7642 | 0.7673 | 0.7704 | 0.7734 | 0.7764 | 0.7794 | 0.7823 | 0.7852 |
0.8 | 0.7881 | 0.791 | 0.7939 | 0.7967 | 0.7995 | 0.8023 | 0.8051 | 0.8078 | 0.8106 |
0.8133 |