1. Place 20 objects on the overhead.





6. You should subtract that number from 9 and tell her how many objects she has in her hand.

Why does this work?

After removing 1 to 10 items from the 20 objects you started with, you will have a number from 10 to 19.

When you add the 2 digits together and subtract from the number 10 to 19, you will always get 9.

< So, when your student gives you some of the items remaining, you need only to subtract from 9 to find how many items she has.