Susan and Tom want to know the combined amount of money they earned last week. They both earned $9 per hour, but Susan also earned a $45 bonus. Susan added her earnings to Tom's earnings to write the expression (9S + 45) + 9T, where S is the number of hours she worked, and Tis the number of hours Tom worked. Tom added their combined hourly wages to Susan's bonus to write the expression 9(S + T) + 45. Which of the following statements is true?