PLZ IF RIGHT I WILL GIVE BRAINLIST: One year, the population of a city was 64,000. Several years later it was 53,120. Find the percent decrease. Ans=%

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! )