The three numbers 2, 4 and 6 have a mean of 4. A fourth number, 12, is added to the set. What is the mean of the four numbers?

Explanation

The first three numbers total 12, so adding 12 makes the total 24, and 24 divided by 4 numbers gives a mean of 6. Averaging the old mean with the new number gives 8, but that treats a single number as though it carried the same weight as the three numbers it joins.