Twenty percent, which is a standard tip, is easy to calculate. Just calculate 10% by moving the decimal one place to the left, then double it for 20%. So 10% of $50 is $5; double that and you have a 20% tip of $10.
My cousin was telling me that she was in Boston several months ago. She got in a conversation with her server in a restaurant. She asked her to tell her honestly what they think of Southerners. The server told her that they groan when Southerners come in the restaurant because they are rude and leave bad tips. I was surprised by that, but after reading the responses to this thread, I realize that the concept that tipping is optional is more widespread than I thought. So much for Southern gentility.