Centerville has a population of 13,000 residents. People under the age of 18 make up 19% of the population of
the town. How many people under the age of 18 live
in Centerville?

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! )