On four winter mornings the temperature was -2, 0, 3 and 7 degrees Celsius. What is the mean of these four temperatures? Enter the number of degrees Celsius.

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The four readings total -2 + 0 + 3 + 7 = 8 degrees, and dividing by the 4 mornings gives a mean of 2 degrees. Treating the reading of -2 as though it were 2 raises the total to 12 and the mean to 3, but a temperature below zero pulls a total down rather than up.