Subtracting Tens and Ones

Subtracting works the same way. Take rods from rods and cubes from cubes.

An example

68 − 25. Tens: 6 take 2 leaves 4. Ones: 8 take 5 leaves 3. The answer is 43.

This works when there are enough

It only works while each digit on top is big enough. The next lesson handles the rest.

Taking away one column at a time

For 57 - 23, take the tens away: 50 - 20 = 30. Then the ones: 7 - 3 = 4. Put them together: 34. As with addition, tens come off tens and ones come off ones, because they are different-sized units.

Checking with addition

If 57 - 23 = 34, then 34 + 23 should give 57 back. It does. This works because subtraction and addition are the same fact family seen from different sides — the idea you met in Chapter 1, now doing real work.

Subtraction does not flip

You can add 34 + 25 or 25 + 34 and get the same total. You cannot do that with subtraction: 57 - 23 and 23 - 57 are not the same. The bigger number goes first when you are taking away. This is worth stating plainly, because addition's flexibility makes children expect it everywhere.