Bone-shaped treats are 5 for $3.50 at shelley’s store and sarah wants to buy 12. she set up and solved this proportion to find how much they would cost. describe her error and find the correct solution.

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! )