A square playground is covered with pebbles, except for two equal sized triangular
portions that have grass. How many square
feet do the pebbles cover?

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! )