Ashopper seeking a bargain combined a 25% off coupon with the store's existing 25% off sale, and brought enough money to cover 50% of the base price. why did this shopper go home disappointed?

With the store`s existing 25% off sale, the price is 75% of the base price. With a 25% off coupon:
0.75 * 0.75 = 0.5625
The new price is 56.25% of the base price. And because a shopper had enough money to cover just 50% of the base price, he was disappointed.