This one is fairly simple once you know what to do.
Remember that with percentages, 100% =1. So when they say 20%, that also means 0.2. First you will need to find what 20% of your total bill is. Once you find that, you can't forget to add it to the original bill to find your final answer. See below for an example:
If the total bill is $29.95 and you want to leave an 18% tip, how much money would you leave?
29.95 x 0.18 = 5.391, so my tip would be $5.39. Now I have to add that to my bill. $29.95 + $5.39 = $35.34
Hope this helps!