To calculate 20% off $20, we can use the formula:

Amount Saved = Original Price x Discount % / 100

Substituting the values, we get:

Amount Saved = $20 x 20 / 100 = $4

Therefore, the amount saved is $4. To calculate the sale price, we can subtract the amount saved from the original price:

Sale Price = Original Price - Amount Saved = $20 - $4 = $16

So the sale price of an item that costs $20 with a 20% discount is $16.