Suppose you have a data set that includes the values 20, 30, and 50. If you add a fourth value to this data set, and after doing that the mean of the data set is 50, what was the value that you added?

The four values must have a total of 4 × 50 = 200 for their mean to be 50. The three original values add to 20 + 30 + 50 = 100, so the added value must be 200 − 100 = 100. The correct choice is 100.

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