A standardized exam's scores are normally distributed. In a recent year, the mean test score was 1521 and the standard deviation was 314. The test scores of four students selected at random are 1920, 1290, 2220, and 1420. Find the z-scores that correspond to each value and determine whether any of the values are unusual

A score of 1920 has a z-score of 1.27.

A score of 1290 has a z-score of -0.74.

A score of 2220 has a z-score of 2.23.

A score of 1420 has a z-score of -0.32.

The score of 2220 is more than two standard deviations from the mean, so it is unusual.

Step-by-step explanation:

When the distribution is normal, we use 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.

If X is 2 or more standard deviations from the mean, it is considered unusual.

In this question, we have that:

Score of 1920:

X = 1920. Then

A score of 1920 has a z-score of 1.27.

Score of 1290:

X = 1290. Then

A score of 1290 has a z-score of -0.74.

Score of 2220:

X = 1290. Then

A score of 2220 has a z-score of 2.23.

Since it is more than 2 standard deviations of the mean, the score of 2220 is unusual.

Score of 1420:

A score of 1420 has a z-score of -0.32.