The answer is 20 (the mean is 20).
When finding the mean (average), you have to add up all the numbers. (So 18+15+22+18+25+19+23+20 which equals 160). Then, you have to divide that number by the total number of numbers in the data set. (So 160 divided by 8, which equals 20). So, the mean of this data set is 20.