To determine how many boxes she will need, first step is to calculate how many books Miss Dennison has in total, that is the math books + the science books:
[tex]24+27=51[/tex]She can only pack 9 books per box, to determine how many boxes she needs you have to divide the number of books by the number of books that fit each box:
[tex]\frac{51}{9}=5.67[/tex]This means that she will fill 5 boxes with 9 books and a sixth one will have only 6 books.
She will use a total of 6 boxes.