It is odd.
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21 is an odd composite number
an odd number
21+9=20
5+5+5+5+1 That is only 5 numbers being added. There can be no answer for six numbers for the following reasons: The numbers are all odd. Two odd numbers make an even number. Two even numbers make an even number. 21 is odd. Take 6 odd numbers and add them up in pairs. Each pair of odd numbers gives an even number so there are 3 even numbers. Even + Even + Even = Even. So sum of any six odd numbers is even but 21 is not even.
The difference of an odd number and an even number is an odd number.