**STRIP BOARD (including doubles)**

e. __The Combinations of One Number__

**Control of error**: Control Chart I. The child looks all the combinations he has made, noticing the tens in red on the diagonal.

**Materials**:

…addition strip board and strips

…paper

…Control Chart I

**Exercise**:

Let’s see all of the different ways to form 10. The blue strip for one is placed on the board. What do we need to make 10? The pink strip for 9 is added, 1+9=10. The child continues in order, making combinations until 1+9=10. The child then writes the combinations and sums on his paper.

Now the child observes that the pink strip decrease in size as the blue strips increase. Also it is observed that 9+1 is the same as saying 1+9. The 9+1 are held up to the 1+9 strips to compare. If I remember that 1+9=10, then I also remember that 9+1= 10. We can eliminate one of these combinations. The strips are put back in their places and 9+1=10 is crossed off the list. We can do the same for 8+2. It is the same as 2+8. This continues until only five combinations remain. It is sufficient to know these five combinations to know the combinations which form 10. The same is done for all numbers 2-18.