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Dear NRICH,

we have had a grat time doing this short challenge. We have come to a conclusion. Here goes:

Firstly,here is the formula: (N-1) X N (N equals number).

Secondly,the answer will always be even.

We foun out the formula by using 9. So 9x9=81 81-9=72
But you may as well do 9x8 because you take away nine afterwards.

This also works for even numbers eg: 10x10=100 100-10=90
Effectively though, you're doing 10x9=90.

Thanks.