To find a certain percentage of a number, just take that number - move the decimal place 2 places to the right (what you are actually doing is dividing by 100), and then multiply the two numbers together.
For example, 40% 0f 88 can be worked out like this:
40 divided by 100 is .44
So:
.44 * 88 = 38.72
And then this means 40% of 88 is 38.72
For your question, all you need to do is:
20/100 = .20 or .2
500,000 * .2
This gives you 100,000.
So 100,000 is your answer!