An independent third party found the cost of a basic car repair service for a local magazine. The mean cost is $217.00 with a standard deviation of $11.40. Which of the following repair costs would be considered an "unusual" cost?

A.

$233.87

B.

$221.65

C.

$192.53

D.

$197.24

In this problem, the approach to solve the unusual value is to assume that the distribution used is a normal distribution. A normal distribution assumes that the mean is equal to zero, the area under the curve is 1 and that the standard deviation is equal to 1. In this case, we use the given mean and standard deviation to determine their corresponding ranges.

First range: mean + - SD

x1 = 217 + 11.4 = 228.4

x2 = 217 - 11.4 = 205.6

This only eliminates one option hence we move next to 2SD

Second range: mean + - 2SD

x1 = 217 + 2*11.4 = 239.8

x2 = 217 - 2*11.4 = 194.2

This leaves option C not included in this range, hence C is the answer to this problem.