To calculate the sale price of a $15 shirt that is 20% off, first find the amount of the discount:
Discount = Original Price × Discount Rate
Discount = $15 × 0.20 = $3.00
Now subtract the discount from the original price to find the sale price:
Sale Price = Original Price - Discount
Sale Price = $15 - $3.00 = $12.00
So, the sale price of the shirt is $12.00.