C'mon, 15% is easy math, and I'm no math whiz. Just figure 10%, double it for 20%, split the difference and add to the 10%..
To get to 15%, I prefer take the bill and divide it by 20. Square that number and divide by 5. Then multiply by 3. Works every time the bill is exactly $100.
I tip 20, but I hate it. Tipping used to be a thank you for excellent service. It was commonly 10% in the 80's, and a 15% tip was EXCITING for a waitress. I wish it would go back to tipping being based on service.