The trouble with percentages (and a major source of math anxiety when people are doing mental arithmetic) is that they are inherently ambiguous. The question most people are not trained to ask is percent of what. Most of the problems occur because when percentages are use the percent of what is not explicitly specified.
In this question a 20% discount probably means deduct 20% from the stated price. But, unless this is stated very explicitly there is still room for confusion. If it really is 20% off the stated price then the correct result is price*0.8.
But it is a little ambiguous. Here is an example. In the UK we have a sales tax called VAT levied at 20%. But 20% of what? Legally it is 20% of the pre-tax price but shops have to quote the total price after tax has been applied (something the USA ought to mandate to avoid confusion for foreigners if not locals as well). So if a discount is described as "we pay your VAT" it sounds like a 20% discount to many but is actually a reduction of ~16.7% on the stated price (1/1.2 since the stated price is pre-tax price*1.2).
So, if you want to avoid confusion with percentages, always ask percent of what?
And, if you are going to be a good scientist always specify explicitly what your percentages mean. If your drug "reduces deaths by 20%" be very explicit in saying from what base (and tell us what percentage of people die without the drug so we can judge the absolute risk: 10 deaths reduced to 8 is a significant result when only 100 people were in the trial but not much when there are 10,000 people in the trial).