If you can't figure out a basic 20 percent tip at a restaurant (take 10 percent and add it to itself. Be generous and always round up) then you might want to revisit your elementary math skills. If you think tipping is optional then you need to revisit your basic being-a-decent-human-being skills and also, read this.
However there are times, such as at the end of a long dinner with a large group of friends, when figuring out a tip requires more advanced math skills or sobriety than you currently have. In these situations, Siri can do the math for you.