The grades of a science test are normally distributed. Thanh finds that his grade on the test has a z -score of negative 2 point 5 Which statement must be true?

A z-score tells how many standard deviations a value is from the mean. Since Thanh’s z-score is −2.5, his grade is 2.5 standard deviations below the mean. It does not mean his grade is exactly 2.5 points below the mean, because a standard deviation is not necessarily one point.

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