1. Place 20 objects on the overhead.
2. Have a student remove any number of items from 1 to 10.
3. The items that remain will be a two-digit number.
4. Have a student find the sum of these two digits and remove that many more items from the projector.
5. Have a student give you some of the remaining objects.
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.