Note: each cell is found by adding the row and column header. For instance, the '8' in row 3, column 5 is found by adding 3 and 5. So 3+5 = 8. The other cells are found this way.
You can see that there are 4 copies of '5' and 2 copies of '11'. So there are 4+2 = 6 copies of what we want (which is either 5 or 11).