Another home business has an average income of $1.200.00 per month with a standard deviation of $100.00. if the income in a given month is $1,030.00, what z-score corresponds to this value?

The equation for the determination of the z-score is shown below.
                                 z-score = (x - μ) / σ
where x is the value in which we need to determine the z-score, μ is the mean or the average, and σ is the standard deviation. 
Substituting the known values,
                                 z-score = (1,030.00 - 1,200) / 100  = -1.7
Therefore, the z-score of 1,030 is -1.7. 

What mathematical statement is missing ? ? the answer is b