Learning Age: 7 to 11 Years

Learning Track: Numeracy Part 2

Mental Maths: A Review of 'Secret Tricks'

T hese neat tricks will help your child change sums to make them easier so they can do them in their head !

Trick 1: Change Order of Adding-up

Remind you child that adding can be done in any order. You can easily show this to your child with a few objects like sweets or blocks.

For example, 3 + 4 is the same as 4 + 3.

Explain to your child that knowing 3 + 4 means that they also know 4 + 3 !

One way that this can help with Mental Maths is to change the order of a sum so that we have to add less to get the answer. For example, 2 + 9 is the same as 9 + 2 (which is easier as there is less to count on).

Here's another example -

8 + 7 + 2

We can re-order adding to make it easier so...

Re-order as 8 + 2 + 7 (because we know what 8 + 2 is)
We know that 8 + 2 = 10,
...so now we have 10 + 7,
...and that we can do easily too !
So answer is 17.

Trick 2: You Can't Change the Order of Take Away

9 – 1 is not the same as 1 – 9.

This may not be a 'trick' but your child must know that they can't change the order of take away. This can easily shown to your child by demonstrating with some sweets.

Trick 3: Taking Away is the Opposite of Adding

7 + 2 = 9

9 – 2 = 7

You get back to what you started with.

(Do as a diagram)

Subtraction is not a totally different thing from addition; they are opposites. To subtract means to figure out how much more you would have to add to get the whole thing.

5 + 2 = 7
7 – 2 = 5 "we get back to 5"

(Show on a number line or say “I have 7 sweets I am given 2 by X, but then 2 area taken by Y” I am still left with ...)

Trick 4: Adding 9 (Same as Adding 10, then Taking Away 1)

When adding to 9 add 10 instead, then take away 1.

We can do this because 9 is the same as 10 – 1.

Example of using this trick -

What does 8 + 9 = ?

We know that adding 9 is the same as add 10 then taking away 1 so,
8 + 9 is the same as 8 + 10 – 1

If your child is ready tell them about another variation on this trick -

99 + 99 is 100 + 100 – 1 – 1 = 200 – 1 – 1

Trick 5: Adding 8 (Same as Adding 10, then Taking Away 2)

When adding to 8 add 10 instead, then take away 2.

We can do this because 10 – 2 = 8.

Example of using this trick -

7 + 8 is 7 + 10 – 2

Trick 6: Adding 7 (Same as Adding 10, then Taking Away 3)

When adding to 7 add 10 instead, then take away 3.

We can do this because 10 – 3 = 7.

Example of using this trick -

8 + 7 is 8 + 10 – 3

Trick 7: You Can Group up Numbers to Take Away

20 – 7 – 3 – 3 is the same as
20 – 10 – 3
= 10 - 3
= 7

Trick 8: Know Your 'Number Bonds'

(More to be added here)

Trick 9: Easy Way to Count 10's

Show them 100 is made up of 10 lots of 10. Do this by showing that you can count tens on your fingers with each finger being 10 rather than 1.

Trick 10: Change Harder Sums into Easier Sums

a.
16 + 3 =
10 + 6 + 3 = "because 16 = 10 + 6"
10 + 9 =
19

b.
27 + 2 =
20 + 7 + 2 = "because 27 = 20 + 7"
20 + 9 = 29

Trick 11: Build on What You Know

Using facts already known, other sums can be worked out.

Here are some examples -

Because you know that 3 + 4 = 7, you know...
30 + 40 = 70

Because you know that 8 + 2 = 10, you know...
18 + 2 = 20
and you also know...
28 + 2 = 30.

Trick 12: When Adding, Start with the 10's, Not the Units

(To do)

Trick 13: When Adding, Add the Easy Pairs First

When adding, find the easy pairs.

Example: 12+7+8 is easier as 12+8=20, then add 7

Trick 14: Adding 10

Adding ten means jumping up ten (100 Grid). The ones digit stays the same but the ten’s digit increases by one.

Example: 5 + 10 = 15

10 + 7 = 17

Trick 15: Change the Numbers

Here we break a number in the sum into smaller parts to make the sum easier.

For example to work out 57 + 5 ...

We know that 5 is the same as 3 + 2,
so instead of 57 + 5,
we can do 57 + 3 + 2.
57 + 3 = 60, leaving just the 2 to add on, which is easy !
So the answer is 62.

This was why it is important why we took time to learn what the numbers up to 10 are made up of.