Isaiah records the amount of money he earns and spends for a one week period. Below is the data he collected. What does -18.69 mean in the context of the story?

At a pizza shop 70% of the customers order a pizza 25% of the customers order a salad and 15% of the customers order both a pizza and a salad if s customer is chosen at random what is the probability that he or she orders either a pizza or a salad