This is very similar to a magic square. Here though there are no diagonals, so it is just the two rows and two columns (shown in grey text)that must add to the same amount.

Click on a number to select it, then click another to swap them over.

Hint: How many totals is each number used in? Any that are counted twice will increase the grand total of the columns and rows above the simple sum of numbers 1-9.

11 | |||||

1 | |||||

9 | 2 | 3 | 4 | ||

5 | 6 | 7 | 18 | ||

8 | |||||

16 |