A jacket that originally cost $20 is on sale for 75% off. What is the sale price of the jacket? What was the amount of the discount?

The sale price of the jacket is 75% off, which means the customer pays 100% - 75% = 25% of the original price.

25% of $20 is (25/100) * $20 = $<<25/100*20=5>>5.
So, the sale price of the jacket is $20 - $5 = $<<20-5=15>>15.
The amount of the discount is $20 - $15 = $<<20-15=5>>5. Answer: \boxed{5}.