Factor Sum

Given any positive integer n, Paul adds together the factors of n, apart from n itself. Which of the numbers 1, 3, 5, 7 and 9 can never be Paul's answer?
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Problem



Given any positive integer $n$, Paul adds together the distinct factors of $n$, other than $n$ itself.

Which of the numbers $1$, $3$, $5$, $7$ and $9$ can never be Paul's answer?

If you liked this problem, here is an NRICH task that challenges you to use similar mathematical ideas.