The Pizza Problem- Math Teaser?

Pizza Galore has a tuesday special where each patron ordering a dessert gets a certain number of pizza slices free. and Tom went to Pizza Galore on Tuesday. Together they ate 10 slices of pizzas and paid $6 for the extra pieces of pizza. If Tom had been by himself and had eaten 10 slices he would have had to pay $10.50 for his extra pieces. How many free slices does the restaurant give each patron?