Question

Which of the following gives an example of a set that is closed under addition?

The sum of an odd number and an odd number

The sum of a multiple of 3 and a multiple of 3

The sum of a prime number and a prime number

None of these are an example of the closure property.

Answer 1

an odd plus an odd can yield an even
7+7=14
a prime plus a prime can yield a composite
5+7=12
a multiple of 3 and a multiple of 3 will always yield a multiple of 3
3(n) + 3(m) =3(n+m)