Quick mental subtraction skills are essential and we have plenty of practice! Bar models and place value charts can still play an important part in developing these skills.

In Year 3 children will learn to use written methods to subtract two 3-digit numbers. This is extended in Year 4 to subtracting 4-digit numbers. Exchanging will often be needed and it is a good idea to encourage children to ask:

“Do I have enough ones/tens/hundreds to subtract ….?”

If not then an exchange is needed.

One of the hardest ideas to get across to children is when they are carrying out a subtraction which requires adjusting, or exchanging across two columns, for example from the hundreds down to the ones if there are no tens in the first number. This only happens when there is a zero involved. For example:

3,703 - 2,286

3 – 6 does not give a positive answer so an adjustment to the top number needs to be made. A ten cannot be moved as there are none.

The procedure is to move a hundred from the hundreds column to the tens column, then a move from the tens column to the ones. Often children will exchange or ‘borrow’ from the hundreds, ignore the zero in the tens and move straight to the ones. They cross out the 7 and make it 6 and make the 3 into 13.

This does not work as it has been 100 exchanged, not 10. Extra steps need to be taken by making the tens column 10 and then taking one ten, making this 9, and adjusting to the ones.

Some children will see a quicker way of making the 700 ten less (690) and adjusting to the ones. Apart from working with very large numbers this is about as hard as it gets for written subtraction, and we have a great selection of practice pages for this.