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Here's a trick for computing a 15% tip in your head. Start by calculating 10% of the bill and then add half. For example, if the bill is $24 dollars, a 15% tip would be $2.40 (10%) plus $1.20 (half of 15%), for a total of $3.60.
Another easy method is to use the sales tax rate in your state. If it the tax rate happens to be around %7 or 8%, the tip should be roughly double the amount of tax that is charged.
Another option is to carry a tip card, a small wallet sized card that has a table on it for calculating tips.