It doesn't matter, you're still very wrong on this and it's easy to verify it ;-)

the total for each row of the matrix is n(n-1)/2

See, this is your problem, you made the same mistake someone down below did ;-)

If you had compared the result of a straightforward solution to your "simplification", you'd have instantly noticed that the result from your side is wrong ;-)

First, this is a question, not an issue ;-)

I think you didn't understand the kata correctly. Maybe you should read the description again, or clarify why/how does that 450 come up.