Solution to the maths puzzle.

The puzzle is a 3x3 matrix.  The fastest way to solve this is to divide 15 by 3 = 5.  Since 5 is a common number, we can do a permutation of 15 below.

1 - 5 - 9

2 - 5 - 8

3 - 5 - 7

4 - 5 - 6

5 which is a common number will be placed in the centre box so that other numbers can be connected to it.

We'll start with 1-5-9 in the middle row.  You can put this in the middle column or diagonal row.  Then we will move on with 2-5-8 and place the 2nd smallest number 2 beside 9.  The trick is to place a small number beside a large number.  Thereafter, you can easily fill up the matrix.