Lucy Brown teaches English in a junior high school. She loves her students very much, and she works hard. She often tells them some interesting stories in class and her students like her, too.
     Yesterday, Lucy found some boys always made faces（做鬼脸）in class. She didn't want them to do that again, so she got all her students together and said, “Boys and girls, when I was young, I liked making faces in class, too. But one day, my grandmother told me not to do that again. She said, "If you make faces again, you will be ugly when you grow up." So you shouldn't do that again.”
Some students thought she was right, but some students didn't. One of the boys put up his hand. “Oh, Tom! What do you want to say?” asked Lucy.
    The boy stood up and said, “Miss Brown, you must regret（后悔）listening to your grandmother now.”

某种商品每件进价为20元，调查表明：在某段时间内若以每件x元（20≤x≤30，且x为整数）出售，可卖出（30﹣x）件．若使利润最大，每件的售价应为____元．