Elaine wants to purchase books. On Monday, she purchased 5 red bags of books and 9 blue bags of books for a total of 78 books. On Tuesday she purchased 7 red bags of books and 4 blue bags of books for a total of 49 books. How many books are in a red bag? How many in a blue bag? 5x+9y=78
7x+4y=49

if the system of equations on above represents what is happening in this problem, define x and y.

Let x= Let y=

What evidence from the system of equations helped you determine which was x and which was y?