Answer:
A. Your battery is likely defective since such poor performance is extremely unlikely.
Step-by-step explanation:
When the distribution is normal, we use the z-score formula.
In a set with mean
and standard deviation
, the zscore of a measure X is given by:
![Z = \frac{X - \mu}{\sigma}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7BX%20-%20%5Cmu%7D%7B%5Csigma%7D)
The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.
If X is more than two standard deviations from the mean, it is considered an unlikely outcome.
In this question, we have that:
![\mu = 600, \sigma = 50](https://tex.z-dn.net/?f=%5Cmu%20%3D%20600%2C%20%5Csigma%20%3D%2050)
Which of the following conclusions is the most appropriate given this information?
Lasted 400 hours, so X = 400.
![Z = \frac{X - \mu}{\sigma}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7BX%20-%20%5Cmu%7D%7B%5Csigma%7D)
![Z = \frac{400 - 600}{50}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7B400%20-%20600%7D%7B50%7D)
![Z = -4](https://tex.z-dn.net/?f=Z%20%3D%20-4)
4 standard deviations from the mean, so unlikely.
So the correct answer is:
A. Your battery is likely defective since such poor performance is extremely unlikely.