Wayne has milk that is 15% butterfat and skim milk without butterfat. How much 15% butterfat milk and how much skim milk should he mix to make 200 gallons of milk that is 13.5% butterfat? 1

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )