The juniors are hosting the next school dance at a local Alief ISD high school. The juniors will earn $3 for each ticket purchased in advance and $4 for each ticket purchased at the door. Based on the space where the dance will be held, the juniors estimate that no more than 300 tickets can be purchased. If 30 people buy advanced tickets, how many people would have to buy tickets at the door for the junior class to have a profit of at least $500?