Choose values. Let y = 100, and let z = 20. Now, evaluate Quantity A. With these values, a $ 5 increase will lead to a bookshelf that costs $ 105. After a 20% discount, the price of the bookshelf will be .8(105) = 84. Now, evaluate Quantity B. With these values, a 20% reduction in the retail price will result in a new price of .8(100) = 80. 80 + 5 = 85. Quantity B is greater.