Kenny plans to mow lawns all summer, and then use the profits to buy video-games and books. He charges $15 per lawn. The video-games are $45 each. The books are $5 each. At the end of the summer he has mowed 35 lawns. There are 5 video-games he really wants, and then he will use the rest for books. How many books can he buy?
Give me a clear explanation.