In homeschool this week, we're learning to estimate things. We haven't learnt much about percentages yet, but the children do know how to divide by five to get that 20% tip if need be. Despite the comments on this post in its original place on facebook, this isn't "Common Core Math." It's a shakedown.
Also... as to leaving a "suggestive tip?" Don't give any money, just halfway unbutton your blouse.