Say an advertiser makes $0.10 every time someone clicks on their ad. Only 20% of people who visit the site click on their ad. How many people need to visit the site for the advertiser to make $20?

Number of people needed to click to make $20:
10 people to make a $1
X 20
200 people to make $20

Those 200 are the 20% that clicked.

(20/100) * x = 200

20 * x = 20000
x = 1000

So 20% of a 1000 people is 200. So a 1000 people.