mean is the average....so you add all the numbers and then divide by how many numbers there were.
12 + 13 + 15 + 28 + 28 +30 + 42 = 168
since there are 7 numbers, you divide 168 by 7.
168/7 = 24
Therefore, 24 is the mean.
The flag is Japan I believe.
Hope this helps!