5 cool math tricks ft. Technicality

For me, the easiest way to calculate a 20% tip is to take 10% of whatever the bill is and then double it. So, using your example, if my bill was $170, I'd take 10% of it, which would equal 17 and then double that number to get 34. That's the tip: $34 and it's super quick and easy to do. 

That said... you really don't even need to calculate a tip nowadays because the waiter or waitress brings over a portable card scanner, so after you've slid your credit card in and entered your PIN, the scanner then asks if you want to leave a tip and gives you the option of using a percentage or a specific number. If you choose the percentage option, you'll get several choices (usually ranging from 10% to 30%) and the machine does the calculation for you. 

At least that's how it's done in Canada. I've noticed in the past that Americans seem to get confused when confronted with the whole chip-and-PIN setup.