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.