The answer is: "42" .
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Step-by-step explanation:
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From the stem-and-leaf plot provided, we can write the following values from the data set, from least to greatest:
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 {15, 18, 46, 50, 50, 50, 50, 57} ;
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To calculate the mean, we take the sum of all the values in the data set; that is, we add up all the values in the data set; and we divide that sum total by the number of values in the data set. 
In our data set, we have EIGHT (8) values. 
To calculate the mean:
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 {15 + 18 + 46 + 50 + 50 + 50 + 50 + 57} / 8 = the mean {our answer}.
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 → {15 + 18 + 46 + 50 + 50 + 50 + 50 + 57} / 8 =
 → {336} / 8 = 42 .
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The answer is: " 42 " .
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 Hope this explanation is helpful.