Selma agreed to buy a used car from Hillary for $2,000. Hillary drew up the contract providing that the exchange would occur the next week. Unfortunately, Hillary was not very good with the keyboard and typed in $200 as the price of the car. Selma noticed that the contract said $200, not $2,000, and was very pleased. She signed it without saying anything. When it was time to make the exchange, Selma gave Hillary $200. In response to Hillary's inquiry regarding an additional $1,800, Selma pulled out the contract and showed her that it said $200. Selma said that there was no mistake on her part because she knew exactly what she was doing and that she thought that Hillary had decided to give her a deal on the car. Which of the following is Hillary's best argument?