Mean, median, mode and range
Mean is the total divided by how many; median is the middle after sorting; range is largest minus smallest.
A worked example
For 6, 13, 19, 26 the mean is 64 ÷ 4 = 16, the median is (13 + 19) ÷ 2 = 16, and the range is 26 − 6 = 20.
The mistake people actually make
Sort the numbers before taking the median. With an even count, the median is the average of the two middle values, not either one of them.