Each day, i use 4 bags of milk for tea. in how much of a day wlll i use 1/2 of a bag of milk?

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! )