A word problem involving Ratios and books.

The key to understanding this problem that was asked to the 9 year olds is a proper model. We use a rectangular modeling set up. In rectangular modeling we always use the smallest unit as the squares that make up the larger rectangles. In this case: Lina's original amount of books. Now, we must focus our attention on the fact that AFTER 24 books are purchased the amount Lina has is twice that of Suling. Suling had 2 rectangles in our original model. Hence Lina MUST have 4 rectangles in our new model as 4 is twice 2. we step back and look at the model. We notice that 3 small rectangles represent the 24 books. Hence the division.