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Yes, there are lots of mental math tricks, and I use many of them. I calculate tips in my head, and even do currency conversions (rounded) in my head when I'm overseas. In my retail days (before the time of electronic cash registers), I computed sales tax and change in my head. (Used to drive some customers nuts that I added the sales tax without looking at the little chart attached to the cash register--but when I showed them that the chart and receipt matched, their eyes just kind of glazed over. They just couldn't believe anyone could do that.)
But I figured that someone looking for a tip calculator probably wasn't interested in figuring out the tip in their head. I've taught my kids these mental math techniques, but in my interactions with their friends I realized it's a dying art. I'm not sure if cash registers that do everything for you are a result of the dumming down of math education, or part of the cause, but many retail clerks these days have a hard time making change even after the cash register calculates it for them. Heck, the clerks at the local Popeye's have a hard time counting out three chicken strips.
And Chillin: Yes, that works fine if you're paying in cash and only care about the total. But if you're paying by credit card, you need to figure out the tip separately.
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