How to help your child understand subtraction

Children need to understand what’s happening to the numbers when they are subtracting. The easy, best practice strategies below are highly recommended. Introduce a new strategy with small numbers that can be easily visualised, particularly if a child has maths difficulties.

Algorithms are now seen as just one way to calculate…not the only way. This is because they do not help children understand anything about numbers. All they do is provide an abstract process. Many children have no idea when they’ve made a mistake because they can’t determine the ‘reasonableness’ of an answer. Try some of these alternatives and notice how your child’s understanding grows.

Subtraction can be thought of as addition

eg 21-18 = ?

If you think of this in terms of addition, then you would ask yourself what you’d have to add to 18 to get 21.  Much easier!

Add or subtract the same amount to each number to make the numbers easier to deal with

By rounding one of them to the nearest 10 or 100, the problem becomes easily manageable.

eg 19 – 4 = ?

Round 19 up to 20 to make it easier. Also add 1 to the number you are subtracting (4) to compensate. The problem becomes:

20 – 5 = ?

= 15

Another example of rounding:

22 – 15 = ?

Round the 22 down to 20. Also take 2 away from 15 to compensate.

20 – 13 = ?

= 7

With larger numbers:

238 – 123 = ?

Round 238 up to 240. Add 2 to 123  to compensate.

240 -125 = ?

= 115

Make small jumps using easy numbers along a number line

The number line can be real or imaginary. A real one is recommended for any child who is having a lot of difficulty with Maths.

It’s easy to draw one up on a piece of paper. Your child may be used to drawing them at school. If so, great. If he doesn’t know how, you can get him to help you to draw one. This will give him practice in numbering and labelling accurately. You can add to it as he starts working with higher numbers. Make sure the ‘easy numbers’ are highlighted, eg 0, 5 10 etc, and all the other numbers are also marked along it.

eg To solve the problem 27-19, you can start at 19 and make jumps to the easy numbers up to 27, or start at 27 and make jumps down to 19.

If you start at 27, the easiest jumps would be 27 t0 25 (=2), then 25 to 20 (=5), then 20 to 19 (=1)

Then add up the jumps: 2+5+1 = 8

If you start at 19, the easiest jumps would be 19 to 20 (=1), 20 to 25 (=5), 25 to 27 (=2)

Then add up the jumps: 1+5+2 = 8

Take away the hundreds, tens and ones in chunks

For children 10 years and older who are familiar with negative numbers on a number line.

eg:

408 – 351 = ?

Take away 300 from 400 (= 100)

Take away 50 (5 tens) from 0 (0 tens) (= -50)

Take away 1 from 8 (= 7)

= 100 – 50 + 7 = 57

eg 51-19 = ?

Take 10 (1 ten) from 50 (5 tens) (=40)

Take 9 from 1 = (-8)

40-8=32

eg 56-32 = ?

Take 30 (3 tens) from 50 (5 tens) (=20)

Take 2 from 6 (=4)

20 + 4 = 24

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