On cold days, Nate likes to make hot chocolate, Ile uses 6 tablespoons of cocoa and 2 tablespoons of sugar Nate triples the recipe when his friends visit for every |blank| tablespoon of cocoa. Nate uses one tablespoon of sugar.
pls help

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! )