It might be interesting to assess your students ability to work with this part-whole model of subtraction by giving them a story problem to solve that deals with the same model. For example, I'm on page 15 of my book. The book has 32 pages. How many more pages do I have left to read? In this problem you may find that some students count up from 15 (15 + X = 32) while others count back from 32 (32 - 15 = X)
You have a great opportunity to connect addtion and subtraction here by comparing students strategies.
