Answer:
see the explanation
Step-by-step explanation:
Let
x ---->the number of books she can sell if she comes back on Sunday
y ----> the number of books sold on Saturday
we know that
The number of books sold on Saturday plus the number of books she can sell if she comes back on Sunday must be less than or equal to the initial number of books in the library (30 books)
so
The inequality that represent this situation is
[tex]x+y\leq 30[/tex]
we have
[tex]y=9\ books[/tex]
substitute
[tex]x+9\leq 30[/tex]
solve for x
subtract 9 both sides
[tex]x\leq 30-9[/tex]
[tex]x\leq 21\ books[/tex]
therefore
The maximum number of books she can sell on Sunday is 21