When it comes time for the tip, you don't want to waste time or pull out the calculator on your phone. To simplify how much you need to give for a tip, move the decimal in your total one place to the left to reach 10%. If your total is $20, you get $2 for a 10% tip. From there you can easily calculate a higher percentage. If you want to do a 15% tip, multiply that number by 1.5 ($3 in our example). For 20%, multiple by two, and so on.
Of course, you do have a calculator in your smartphone, so if you don't mind pulling it out briefly, you can figure out the tip with a single calculation. Multiply your total time 1.[tip%]. So, for a 20% tip, multiply your total by 1.2. This will give you your new total. The difference between your new total and the subtotal will give you your tip amount.