A $15 shirt would cost $12 if it were 20 percent off. The 20 percent discount would come to $3, and subtracting $3 from $15 gives $12.
A discount is generally offered as a percentage of the cost price. In this case, the cost price of the shirt is $15. The number 20 divided by 100 is equal to 0.2, and 0.2 times 15 is equal to 3. The discount can also be calculated by multiplying 0.2 by the cost price before discount. The final selling price of the shirt is obtained by adding tax to the price after discount.