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:
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:
Which of the following conclusions is the most appropriate given this information?
Lasted 400 hours, so X = 400.
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.
