1. those who don`t know them
2. those who know them
3. those who know but never seen them
4. those who know but won`t do them
5. those who know but forget them
6. those who know but never use them
7. those who know but left before using them
8. those who know but went to do better things
9. those who know but too lazy to do them
10. you get the point...=P
More accurately, What, those who DO understand binary, and those who DO not... "can" is fairly irrelevant in the terms of the joke, as pretty much anybody who understands numbers can.