5 Math Tricks to Learn for the NSW Selective Test

The NSW Selective Test gives your child about 70 seconds per maths question. That is not enough time to work everything out the long way. Children who do well on the maths section are not faster calculators — they have a small toolkit of shortcuts they reach for instinctively.

Here are five of the most useful. Each one is short to learn, easy to practise, and shows up on the real test in some form almost every year.

1. The 10% Anchor

What it does: Lets your child find any percentage of a number using only 10%.

Finding 10% of something is easy — just move the decimal point one place to the left. From there, every other percentage is built by halving, doubling, and adding.

Example 1: Find 15% of $400.

• 10% of $400 = $40
• 5% of $400 = half of 10% = $20
• 15% = 10% + 5% = $60

Example 2: A jacket costs $80 and is 35% off. How much do you save?

• 10% of $80 = $8
• 30% of $80 = three lots of 10% = $24
• 5% of $80 = half of 10% = $4
• 35% of $80 = $24 + $4 = $28

The same idea works for 20% (double the 10%), 25% (10% + 10% + 5%), 30% (triple), and so on. Most percentage questions on the Selective test can be answered in two steps once your child has the 10% in their head.

Try these:

1. What is 20% of 70?
2. What is 35% of 200?
3. What is 15% of 60?
4. What is 45% of 80?

Answers: 14, 70, 9, 36

2. The Flip Trick

What it does: Lets your child swap the two numbers in a percentage question to make it easier.

Here is something most adults do not know: a% of b is always equal to b% of a. The numbers can be swapped, and the answer stays the same.

Example 1: What is 8% of 50?

Most children freeze. But flip it: 50% of 8 is just half of 8, which is 4.

Example 2: What is 4% of 25?

4% is awkward. But flip it: 25% of 4 is just a quarter of 4, which is 1. Done in two seconds.

It feels like cheating. It is not — it is just maths. Once your child sees it, they will look for it on every percentage question. Look for one number that is awkward and one that is friendly (10, 50, 100, 25), and flip.

Try these:

1. What is 12% of 50?
2. What is 2% of 150?
3. What is 6% of 25?
4. What is 16% of 25?

Answers: 6, 3, 1.5, 4

3. Keep, Change, Flip (Dividing Fractions)

What it does: Turns fraction division into fraction multiplication, which is much easier.

Dividing fractions is the topic most Year 6 students get wrong on practice tests. The fix is one rule:

Keep the first fraction. Change the divide sign to a multiply sign. Flip the second fraction upside down.

Example 1: 35 ÷ 27

• Keep: 35
• Change: ÷ becomes ×
• Flip: 27 becomes 72

So 35 ÷ 27 = 35 × 72 = 2110.

Example 2: A bag of flour weighs 34 of a kilogram. How many 18 kg portions can you make?

This is 34 ÷ 18. Keep, change, flip: 34 × 81 = 244 = 6 portions.

That is it. There is no other step. Practise it a few times and it becomes automatic.

Try these:

1. 12 ÷ 14
2. 23 ÷ 45
3. 56 ÷ 13
4. A ribbon is 45 of a metre long. How many 110 m pieces can you cut?

Answers: 2, 1012 = 56, 156 = 212, 8 pieces

4. The Butterfly Method (Comparing Fractions)

What it does: Tells you instantly which of two fractions is bigger, without finding a common denominator.

Example 1: Which is bigger, 34 or 57?

Cross-multiply diagonally — top of one fraction with the bottom of the other:

• 3 × 7 = 21 (this number "belongs to" the 3/4)
• 5 × 4 = 20 (this number "belongs to" the 5/7)

21 is bigger than 20, so 34 is bigger than 57.

Example 2: Which is bigger, 58 or 711?

• 5 × 11 = 55 (belongs to 5/8)
• 7 × 8 = 56 (belongs to 7/11)

56 is bigger, so 711 is bigger — only by a hair, which would be hard to see without the trick.

It looks like magic, but it is just the common-denominator method with the unnecessary step removed. For ordering or comparing fractions under time pressure, nothing is faster.

Try these:

1. Which is bigger, 23 or 35?
2. Which is bigger, 49 or 37?
3. Which is bigger, 512 or 38?
4. Put these in order from smallest to largest: 25, 38, 411.

Answers: the larger fraction in each pair is 23, then 49, then 512. Final ordering: 411 < 38 < 25.

5. Watch the BODMAS Trap

What it does: Stops your child from making the single most common mistake in the calculation questions.

Example 1: What is 20 − 5 × 3?

The trap answer is 15 × 3 = 45. The right answer is 5, because multiplication happens before subtraction: 20 − 15 = 5.

Example 2: What is 48 ÷ 6 + 3 × 4?

Scan first. There is a division and a multiplication — both happen before the addition.

• 48 ÷ 6 = 8
• 3 × 4 = 12
• Now add: 8 + 12 = 20

The trap answer is to go left-to-right and get 48 ÷ 6 = 8, then 8 + 3 = 11, then 11 × 4 = 44. Wrong.

The rule is BODMAS (Brackets, Orders, Division/Multiplication, Addition/Subtraction). Children who rush left-to-right will fall for the trap every time. The fix is a one-second habit: before calculating, scan the whole expression and circle any multiplication or division. Do those first. Then do addition and subtraction.

This single habit will catch one or two extra marks on almost every Selective test.

Try these:

1. 30 − 4 × 5
2. 6 + 2 × 8
3. 24 ÷ 4 + 2 × 3
4. (8 + 2) × 3 − 5

Answers: 10, 22, 12, 25

How to Practise These

Knowing the trick is not the same as using it. Five sessions of ten minutes each is enough to make these automatic. Try this routine:

1. Day 1: Pick one trick. Watch one example, then do five questions on it. No mixing.
2. Day 2: Same trick. Add five more questions, plus three from yesterday.
3. Day 3: New trick. Same pattern.
4. Day 4–5: Mixed practice — your child has to recognise which trick to use, not just apply one.

That last step is the hard one. On the real test, no one will tell your child "this is a flip-trick question." Recognising the shortcut is half the skill.

Want to See Them In Action?

PrepKit has short video walkthroughs of each of these tricks, plus practice questions tagged by topic so your child can drill the exact pattern they want.

Start a free practice session →

The 2027 NSW Selective Test is on track for early May 2027. If your child is in Year 5 now, the months between June and December are the highest-leverage window for building these habits — well before the pressure of the year of the test.