Mark found the mean of 26, 32, 22, 35, 25 using the strategy below. Why is his strategy valid???

"I guessed the mean was 30. Then I found the difference between each data point and 30 and I got -4, 2, -8, 5, -5. The average of these numbers is -2. So 30 and -2 is 28, which is the average of the original data set."