when you see a "%" you need to ask " a percent of what"?
Here, it's a percent of the original prize, so the discount is 20% of the original prize, that is 20% of 35.
20% is also 20/100 or 1/5, so we can calculate this like this:
[tex] \frac{1}{5} *35=7[/tex]
so the discount is 7 dollars.