Solution: Let's buy X teacups. Then supermarket A can only discount (x-10) pieces when purchasing10 pieces at the original price; Then the cost of supermarket A is expressed as 5 *10+5 * 0.8 (x-10); The cost of supermarket B is expressed as 5*0.9x, so the inequality 5 *10+5 * 0.8 (x-10) < 5 * 0.9x; Get x & gt20, so buy at least 2 1 teacups, and it is more cost-effective to choose supermarket A.