题目

A young officer was teaching some old soldiers. They had been in the army for many years and did not like officers, young or old. They did not think this young officer could tell them anything about how to fight in a war. Private Jones was nearly sixty years old and had fought in many wars. He had a row of medals on his chest. “Imagine you are in a battle,” the young officer said to him. “You see seven hundred enemy soldiers coming towards you. What do you do?” Private Jones thought for a few moments, and then he said, “I shoot them all with my rifle.” “Now imagine there are seve某水果店出售一种水果，每只定价20元时，每周可卖出300只．试销发现：（1）每只水果每降价1元，每周可多卖出25只．设现在定价每只x元（x<20），一周销售收入为y元，则y与x的函数关系式为 ； （2）每只水果每涨价1元，每周将少卖出10只，如何定价，才能使一周销售收入最多？（3）根据以上信息，你认为应当如何定价才能使一周销售收入最多？