The question you opened
A 20 percent tip on 64 dollars
The check is 64 dollars and you want to leave 20 percent.
What is the tip, in dollars?
$12.80
Moving the decimal point one place gives 6.40, which is 10 percent. Double it and the tip is 12.80, leaving a total of 76.80.
Ten percent of any number is that number with the decimal point moved one place left. Ten percent of 64 is 6.40. Twenty percent is twice that, so 12.80, and the total comes to 76.80. The same base covers the whole tipping range. Fifteen percent is 10 percent plus half of it, so 6.40 plus 3.20 is 9.60. Eighteen percent is 20 percent minus 2 percent, so 12.80 minus 1.28 is 11.52. Twenty five percent is a quarter, so 16.
Every common tip is a short combination of a tenth, a half of a tenth, and a doubling. There is a second route worth having when the check ends awkwardly. Round the check first, then adjust. On 63.40, tip on 63 and add a few cents, or simply tip on 65 and stop. Nobody is auditing the difference, and rounding up is what most people do anyway once the arithmetic is easy enough to attempt.
In the United States, tip is customarily calculated on the pre tax subtotal, though many people tip on the total and treat the difference as rounding. On a 64 dollar check the gap is well under a dollar. What matters more is that a tip left in cash reaches the server faster and more completely than one added to a card in many restaurants.
The habit that transfers well beyond restaurants is the decimal point itself. Moving it one place is dividing by ten, two places is dividing by a hundred. Sales tax, commission, service charges, and interest all become one small movement plus a doubling or a halving, and none of them need a phone.
Technique: Move the point, then double