Question

WILL GIVE MEDAL
ben ordered 29 pizzas. he order cheese, peperoni, and supreme. Cheese was $7, Peperoni was $9, and supreme was $11. He spent exactly twice as much on peperoni as cheese. If ben spent $225,how much of each pizza did he buy.
So far I have:
c+p+s=29
7c+9p+11s=225

Question is: how do I set up the last equation for spending twice as much on peperoni as cheese so I can do the elimination process?