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The outlier in the data set is 37.
The mean with the outlier included is:
(21 + 13 + 13 + 37 + 13 + 23 + 25 + 15) / 8 = 19.5
The mean without the outlier is:
(21 + 13 + 13 + 13 + 23 + 25 + 15) / 7 = 17.14