If 300 rich people on Earth have the same amount of wealth as 3,000,000,000 poor people, then 5 rich people would have the same amount of wealth as poor people. Fill in the blank.

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! )