Age 7 to 11
You could start with the numbers in the top row of the matrix. What
two numbers could be added together to make $1$?
Why not write them in, one to the left of the row and one above the
number $1$ in the matrix. Now, you could fill in the rest of the
numbers which will add to make that top row. Test out your numbers
by looking at the second row.
Be prepared to start again if it doesn't work!