The method most people use is to multiply the number you want to know the percentage of, by the percentage, in this case 500 * .20, which will give you the right answer. But if you want a quicker answer all you have to do is use the 100 base method. If for example you want to know what 30% of 1000 is, figure out what 30% of 100 is. The answer of course is 30, too easy huh? Now take 30 and either multiply it by 10 to give the correct answer of 300 or when you are looking for a percentage for the number 1000, all you have to to do is add a zero to the end of the number to get the correct answer.
This method works every time, and if you work at a job where you need to come up with a fractional answer or a percentage in short order, this is something you should become very good at doing. It is also a good thing to know if you are solving complex problems in subjects such as statistics, or any other math discipline where a number of calculations have to be done to come to the answer.
It's easy to figure the percentage of whole numbers if you have a trick or two such as this up your sleeve. Practice this method until you perfect it and you'll be on your way to becoming a number sense champion.