OK, you have ten cans of paint worth 25 dollars a can. this gives you a grand total of 250 dollars worth of paint. Then you have 20 cans worth 30 dollars a piece, giving you a grand total of six hundred dollars. you would then add 250 to 600, giving you 850 dollars worth of paint. Then subtract 500 dollars. this will tell you that 350 dollars worth of paint was used.