An exam has 20 exercises. For every exercise solved correctly you get 8 points and for every exercise solves wrong are taken off 5 points. Ben had 13 points. How many exercises did he solve?

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! )