A college student goes to lunch at school and pays for lunch with her ID. Her mother put money into her account for her to do so, but she doesn’t know how much. ​Lunch is the same cost each day​ , but she doesn’t know what the cost is. After buying ​4 lunches​, she sees that her ​account balance is $91​ . After buying ​9 lunches​ , her ​balance is $67.25​ .