Math Dude's easy trick for calculating a 15% tip. No calculator required!
If you want to leave a 15% tip, use the fact that 15% = 10% + 5%.
Calculate 10% of the bill by moving the decimal point one position to the left. Then, take that number, divide it in half, and add the new amount to the original 10%.
For more, see How to Use Percentages to Easily Calculate Tips
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