On Monday, Dalaya had $15. On Tuesday, she bought a T-shirt for $12. On Wednesday, she found $5.50 on the street. On Friday, she found $3 in her pocket, and then spent $8 on a cheeseburger for lunch.
Write two expressions to represent the changing amounts of money Dalaya has: one using only addition and one using only subtraction.
Determine how much money Dalaya has by the end of the day on Friday. Show your work to justify your answer.