When melons become potatoes...

From Sciencing, How to Calculate Percent Deviation:

Calculate the Mean

Calculate the average, or mean of your data points. To do this, add the values of all data points, then divide by the number of data points. Say you have four melons, with weights of 2 pounds, 5 pounds, 6 pounds and 7 pounds. Find the sum: 2 + 5 + 6 + 7 = 20, then divide by four, since there are four data points: 20 / 4 = 5. So your potatoes have a mean weight of 5 pounds.