Question

One weekend, there were 99 pizzas ordered for the sophomore dorm. That weekend 16 customers ordered their pizza with just pepperoni, 18 customers ordered their pizza with just sausage, 19 ordered theirs with just onions, 12 ordered theirs with pepperoni and sausage, 13 ordered theirs with sausage and onions, 5 ordered theirs with pepperoni and onions, and 6 ordered theirs with all three items. The remaining pizzas were cheese pizzas with no toppings. How many customers ordered at most one topping on their pizza?