Some additional context would be helpful in providing a complete answer. But the basic idea is this:
• Object ABC has a price X set on it.
• Hey, ABC is on sale! Let's say there's a 20% discount. That means 20% of the price X is subtracted from X.
• Boo, there's a 15% sales tax! This means whatever you would pay for ABC, discount included, you will ultimately have to pay an additional 15% of, or 0.15 times, that amount in tax.
Suppose you're looking to buy a T-shirt. The price tag on says it costs $25, but there's a 20% discount and a 15% sales tax.
• The discount has a value of 0.2 ($25) = $5. Subtract this from the original price, and its discounted price is $20.
• The sales tax is applied to this discounted price, and comes out to 0.15 ($20) = $3. This gets added to the discounted price, so you end up paying $23.