<span>The mean, or average, of a data set
is calculated by summing all of the numbers in the data set and
dividing that sum by the number of data points in the set. If the mean
of a data set is known, then that mean contains information on the
missing number. Multiplying the mean by the number of data points gives
the "true" sum of all the data points, including the missing number.
The sum of the known data points is less than this true sum, because
the sum of the known points does not include the value of the missing
number. The difference between the sum of the known data points and the
true sum is exactly the value of the missing number. However, if
multiple data points are missing, this method does not work.</span>
