I love Tom Lehrer dearly, but I don't see how it's appropriate here. Do you think that trying to teach kids to understand what they are doing is wrong? Nowhere am I suggesting that I have the impression that the child writing down "When you add two even numbers you always get 7" would be considered a valid answer on this worksheet - clearly they're trying to get kids to comprehend parity.

Or are you suggesting that the answer would be different if you were missing two thumbs?