A data set contains the values 4, 5, 6 and 45. What is the mean of all four values, and what is the mean once the value 45 is removed?

Explication

All four values total 60, and 60 shared between 4 values is a mean of 15; taking 45 out leaves a total of 15 shared between 3 values, which is a mean of 5. Taking 45 away from the total but still dividing by 4 gives 3.75, and that is the standard slip, because removing a value reduces the count as well as the total and both parts of the division have to be worked out again.