In the following sudoku, within each and every 3x3 ‘box’, the rows add up to 9, 15 and 21 and the columns add up to 9, 15 and 21 (in some order).

For instance one box might be:

2 3 4

7 5 9

6 1 8

as the row totals are 9, 21 and 15, and the column totals are 15, 9 and 21.

Other than that, usual sudoku rules apply: the numbers 1 to 9 appear once each in every row, column and box.

Starting tip: list all the possible ways of totalling 9 then look at the central box.