The gymnasium at a local high school has 1000 seats. The state basketball championship game will be held there next Friday night. Tickets to the game will be priced as follows: $\$ 10$ if purchased in advance and $\$ 15$ if purchased at the door. The booster club feels that at least 100 tickets will be sold at the advance ticket price. Total sales of at least $\$ 5000$ are expected.
(a) Use $x$ to denote the number of tickets sold in advance. Use $y$ to denote the number of tickets sold at the door. Write a system of linear inequalities that describes the possible numbers of tickets sold at each price.
(b) Graph the region that represents all possible combinations of ticket sales.