A previous node suggested the following:
Add all the numbers, then divide by the number of numbers. Average.

Technically, this is correct, but we should note that the mean for a sample is usually considered slightly differently. Sure, it is still the sum of values divided by the number of cases. However, given degrees of freedom problems, one must subtract 1 from the number of cases when using sample means in order to arrive at an unbiased value. When used correctly, the mean is what is known as BLUE, which is the Best Linear Unbiased Estimator.