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.