Dolly is a college student who works as a part-time server in a restaurant. Her usual tip is 20% of the price of the meal. A customer ordered a piece of pie and said that he would appreciate prompt service. Dolly abided with the customer's request. The customer's bill was $8, but the customer left a $100 bill on the table and did not ask for a receipt. Dolly gave the cashier $8 and pocketed the $100 bill. Dolly concludes that the customer thought that he has left a $10 bill, although the customer did not return to correct the apparent mistake