52. Break-Even Audra wants to establish a lemonade stand on her corner. Her father buys some lumber and other materials and makes Audra a lemonade stand for $40 . Audra goes to the store and buys ingredients for lemonade. She determines that each cup of lemonade will cost her $0.03 to make, but the actual cup will cost her an additional $0.07. She decides to sell the lemonade for $0.30 a cup . (a ) Find the revenue function R, treating the number of cups of lemonade r as the independent variable. (b) Find the cost function Ctreating the number of cups of lemonade x as the independent variable. (c) Graph the revenue and cost functions in the same Cartesian plane . (d ) Find the break -even number of cups of lemonade that Audra must sell. What are the revenue and cost at this number of cups of lemonade Label the point on the graph drawn in part ( c).