It took David 2/3 of an hour to bicycle 12 miles from his house to his friend's house. He followed the same route home. His average rate on the ride home was 3 mph faster than on his ride out. What was David's average rate on the ride home

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! )