Calculate 13 minus four.
In the black box, we have 13 blocks. We have one block of 10 and three blocks of ones, 13. Let’s try to take away four of the blocks: one, two, three, but now we have a problem. We have taken three blocks away from the thirteen, and there is now one block of 10 inside our box.
How do we take one away from this block of 10? We have to turn this block of 10 into ones. Instead of having one large block of 10, we need 10 small blocks, like this. Now that we have 10 small blocks, we can take one more block away.
Now we have nine blocks in the box. If you take four away from 13, you get nine. 13 minus four equals nine.