Find the sum of all the values in the data set. For example, in the data set {10, 11, 12, 13}, the sum is 46. (10 + 11 + 12 + 13 = 46)
Count the number of values in the data set. For example, there are four values in the data set {10, 11, 12, 13}.
Take the sum (46) and divide this quantity by the number of values in the data set (4). For example, 46 / 4 = 11.5. This is the mean -- 11.5
Check your answer by performing steps 1 to 3 again.