Question

a group of high school jrs sells lemonade for their class. when they sell each cup for .30 they sell 215 cups a day. the jrs try to make more money so they raise the price of the lemonade. However, each time they raise the price of a cup of lemonade by ten cents, the lose five customers.

2 equations :
(.30+.10X)(215-5X)
Revenue=price x quanity

Question: how much should the jrs sell each cupof lemonade for if they want to maximize their revenue??
i know you use a calculator to do it i just forgot how?!