Sean says that to add a number to -100 and still have -100 is to add zero. candice says that she can add two numbers to -100 and still have -100. who is correct and why ?

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! )