The statement 3+3=6 serves as a/an what to the conjecture that the sum of two odd numbers is an odd number

A proposition states a fact.
If we can only find even a single one situation that satisfies all the conditions of the proposition, but which turns out to be false, it immediately invalidates the proposition.
This situation is an example of a "counter example".

If the proposition is odd+odd=odd, then
3+3=6 is a counter example of the proposition.

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Subtracting is the opposite of adding