The best I've heard is "How many quarters are in the game", spoken by a Blonde at a football game.

>>I was at the checkout of a K-Mart. The clerk rang up $46.64 charge. I Gave her a fifty dollar bill. She gave me back $46.64. I gave the money back to her and told her that she had made a mistake in MY favor. She became indignant and informed me she was Educated and knew what she was doing, and returned the money again. I gave her the Money back... same scenario! I departed the store with the $46.64.
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>I've seen a few... a girl who measured 4 2/3 yards of fabric, and couldn't find it on the printed table taped to the table how much was it in decimal yards; it took her a few minutes but she eventually located it (.667 - even Walmart was generous enough to hire someone who knows proper round(x,3)). Then she forgot the four and typed .667...
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>Or a guy who asked me how many cigarettes do I get out of one bag. "Didn't count, but one cigarette should be 1 gram, it says 200 grams on the bag, so that should be 200 cigarettes". "Wow! How did you calculate it so fast? I never got the hang of those metric units." (at least he was a customer)