That would be 40 books on each shelf. Hope that helped :)
Here is the process:
2,160/ 9 stands = 240 books per stand
240 stands/ 6 shelves each = 40 books on each shelf.
You would need 3 can of spray paint.
3 dozen cookies = 36 cookies
Brother ate 0.25(36) or 9 cookies.
36 - 9 = 27 cookies remain.
Sara took (1/3)(27) or 9 cookies went to her neighbor.
There are now 18 cookies of which Sara ate 4.
So, 18 - 4 = 14.
There are 14 cookies left.
24=3(n-5)
Distribute.
24=3(n) + 3(-5)
24 = 3n -15
Add 15 to both sides.
39 = 3n
Divide both sides by 3.
n = 13.
I hope this helps!