Put the numbers 123456789 in the bubbles so that each edge adds up to the same number?

To solve this problem, we need to find the magic constant, which is the sum of each edge in the completed square. The formula to calculate the magic constant for a 3x3 magic square is (n^3 + n) / 2, where n is the number of cells on each side. In this case, n=3, so the magic constant is (3^3 + 3) / 2 = 15. By arranging the numbers 1-9 in the square such that each edge sums up to 15, we get the following arrangement:

<code>  8  1  6
3  5  7 4  9  2
</code>