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posted by mary .
Eric has computed that it takes an average (mean) of 17 minutes with a standard deviation of 3 minutes to drive from home, park the car, and walk to his job.
One day it took Eric 21 minutes to get to work. You would use the formula for transforming a raw score in a sample into a zscore to determine how many standard deviations the raw score represents.
Since his "score" is 21, you would subtract the mean of 17 from 21 and divide that result (4) by the standard deviation of 3.
The zscore of 1.33 tells you that Ericâ€™s time to get to work is 1.33 standard deviations from the mean. Another day, it took Eric only 12 minutes to get to work. Using the same formula, determine the z value. Is it positive or negative? Explain why it should be positive or negative

What happens when you subtract 21 from 13?

I do not understand your answer to my question