Answer:
C. $5180
Step-by-step explanation:
To solve this question, we have to understand the normal probability distribution and the central limit theorem.
Normal probability distribution:
Problems of normally distributed samples can be solved using 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.
Z-scores lower than -2 or higher than 2 are considered unusual.
Central limit theorem:
The Central Limit Theorem estabilishes that, for a random normally distributed variable X, with mean and standard deviation , the sample means with size n can be approximated to a normal distribution with mean and standard deviation 
In this problem, we have that:
Which of the following mean costs would be considered unusual?
We have to find the z-score for each of them
A. $6350
By the Central Limit Theorem
Not unusual
B. $6180
Not unusual
C. $5180
Unusual, and this is the answer.
