Why is it not a system of linear equations?

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! )

Is a square always, sometimes, or never a parallelogram

Zahra runs a 500-meter race at a constant speed. which graph shows her distance from the finish line during the race?