20 Mental Math Tricks for Kids, Explained Step by Step

By Mauro Ferrante · Updated · 9 min read

Most adults do arithmetic in their heads with a handful of shortcuts they picked up without noticing: rounding to a friendly number, adding ten and taking one back, doubling and halving. Children are rarely taught these on purpose. They are taught the written method, column by column, and left to discover the shortcuts on their own, or not.

This guide lists the twenty tricks taught in Math Ninja, a free game on this site, in the order the game teaches them. Each one has a worked example and, more importantly, the reason it works. A trick a child can explain is a trick they will still have at forty. Every example below is the one the game uses in its Watch phase, so a parent reading this and a child playing the game are looking at the same numbers.

How to use this with a child. Pick one trick. Say the steps out loud together three or four times with different numbers. Then ask them to explain it back to you. If they can say why it works, move on. If they can only say what to do, stay a day longer.

➕ Addition

1. Friendly Numbers

Round a tricky number to a ten, add, then give back the extra.

Example: 48 + 35 = 83

  1. 48 is so close to 50. Round it up by 2. 48 + 2 = 50
  2. Now the sum is easy. 50 + 35 = 85
  3. We borrowed 2 to round up. Give it back. 85 − 2 = 83

Why it works: adding 2 to one number and taking 2 off the answer cancels out. We just moved the awkward part to the end, where it is a single easy step.

2. Left to Right

Add the tens first, then the ones, then join them.

Example: 53 + 36 = 89

  1. Tens first: 50 and 30. 50 + 30 = 80
  2. Now the ones: 3 and 6. 3 + 6 = 9
  3. Join them. 80 + 9 = 89

Why it works: addition can happen in any order. Tens plus tens, then ones plus ones, is the written method turned sideways, and it keeps the big numbers in view so the answer is roughly right from the first step.

3. Making Tens

Borrow just enough from one number to make the other a ten.

Example: 8 + 7 = 15

  1. 8 needs a little to make 10. How much? 10 − 8 = 2
  2. Take 2 from 7. 7 − 2 = 5
  3. Ten and 5. 10 + 5 = 15

Why it works: 8 + 7 and 10 + 5 are the same total; we only moved 2 from one pile to the other. Ten is easy to add to anything, so the whole job becomes finding how much a number needs to reach ten.

4. The Plus 9 Slide

Adding 9? Add 10 and slide back one.

Example: 67 + 9 = 76

  1. Add 10 instead, that's easy. 67 + 10 = 77
  2. Slide back one. 77 − 1 = 76

Why it works: 9 is one less than 10. Adding 10 is a single digit change, so add the easy number and correct by one.

5. Doubles Plus One

Neighbours? Double the smaller one and add 1.

Example: 7 + 8 = 15

  1. Double the smaller number. 7 × 2 = 14
  2. Add one more. 14 + 1 = 15

Why it works: neighbouring numbers are a double plus one. Children tend to know their doubles cold, so 7 + 8 becomes a fact they already own plus one.

➖ Subtraction

6. Add Up to Subtract

Count up from the small number to the big one, like a shopkeeper giving change.

Example: 100 − 84 = 16

  1. From 84, jump to the next ten, 90. How far? 90 − 84 = 6
  2. From 90 up to 100. 100 − 90 = 10
  3. Add the two jumps. 6 + 10 = 16

Why it works: the gap between two numbers is the same whichever way you walk it. Counting up in friendly jumps avoids borrowing altogether, and it is exactly how a shopkeeper counts change.

7. Shift Both Numbers

Move both numbers by the same amount so there is no borrowing.

Example: 72 − 39 = 33

  1. Add 1 to both. 72 becomes… 72 + 1 = 73
  2. …and 39 becomes a round ten. 39 + 1 = 40
  3. Same gap, easier numbers. 73 − 40 = 33

Why it works: adding the same amount to both numbers leaves the difference unchanged (73 and 40 are as far apart as 72 and 39). We chose the shift that turns the bottom number into a round ten.

8. All from 9, Last from 10

Taking from 1,000? Each digit from 9, the last one from 10.

Example: 1000 − 436 = 564

  1. Hundreds digit from 9. 9 − 4 = 5
  2. Tens digit from 9. 9 − 3 = 6
  3. Last digit from 10. 10 − 6 = 4
  4. Read the three answers as one number. 5 6 4 = 564

Why it works: 1000 = 999 + 1. Subtracting from 999 never needs borrowing (every digit is a 9), and the extra 1 is handled by taking the last digit from 10 instead of 9.

9. The Minus 9 Loop

Taking 9? Take 10 and give one back.

Example: 54 − 9 = 45

  1. Take 10 instead. 54 − 10 = 44
  2. That was one too many. Give it back. 44 + 1 = 45

Why it works: 9 is one less than 10. Taking 10 is easy, but that removes one too many, so put one back.

10. Drop to a Ten

Take away in two easy pieces: first down to a ten, then the rest.

Example: 83 − 15 = 68

  1. First take 3 to land on a ten. 83 − 3 = 80
  2. How much is still left to take? 15 − 3 = 12
  3. Now take the rest. 80 − 12 = 68

Why it works: subtracting in two steps is the same as subtracting all at once. The first step lands on a round ten, and taking things away from a round ten is easy.

✖️ Multiplication

11. Double and Halve

Halve one number, double the other. Same answer, easier sum.

Example: 14 × 15 = 210

  1. Halve 14. 14 ÷ 2 = 7
  2. Double 15. 15 × 2 = 30
  3. Multiply the easy pair. 7 × 30 = 210

Why it works: halving one factor and doubling the other leaves the product unchanged (14 × 15 and 7 × 30 are both 210). It turns an awkward pair into a pair with a round number in it.

12. The 11 Rule

Two digits times 11: put their sum in the middle.

Example: 35 × 11 = 385

  1. Add the two digits, 3 and 5. 3 + 5 = 8
  2. Put that in the middle: 3 _ 5. 3 8 5 = 385

Why it works: 35 × 11 = 35 × 10 + 35 = 350 + 35. Writing that addition digit by digit puts 3 in the hundreds, 3 + 5 in the tens and 5 in the ones. When the two digits add to more than 9, carry the 1 into the first digit.

13. Times 5

Halve the number, then times 10.

Example: 24 × 5 = 120

  1. Halve it. 24 ÷ 2 = 12
  2. Times 10, just add a zero. 12 × 10 = 120

Why it works: 5 is half of 10. Halving first and then adding a zero does the same job as multiplying by 5 in one step, without any tables.

14. Times 9

Times 10, then take one group away.

Example: 7 × 9 = 63

  1. Times 10 first. 7 × 10 = 70
  2. Take away one 7. 70 − 7 = 63

Why it works: 9 groups is 10 groups minus one group. Times ten is a zero on the end, then take one copy of the number away.

15. Double-Double (×4)

Times 4 is just doubling twice.

Example: 18 × 4 = 72

  1. Double it. 18 × 2 = 36
  2. Double it again. 36 × 2 = 72

Why it works: 4 = 2 × 2. Doubling twice multiplies by four, and doubling is something children do without thinking.

16. Times 15

Times 10, add half of that.

Example: 22 × 15 = 330

  1. Times 10. 22 × 10 = 220
  2. Half of that. 220 ÷ 2 = 110
  3. Add them together. 220 + 110 = 330

Why it works: 15 = 10 + 5, and 5 is half of 10. So times 15 is times 10 plus half of that.

17. Squares ending in 5

First digit times the next number, then stick 25 on the end.

Example: 35 × 35 = 1225

  1. Multiply 3 by the next number up, 4. 3 × 4 = 12
  2. Attach 25 to the end. 12 25 = 1225

Why it works: (10n + 5)² = 100·n·(n+1) + 25. The algebra is for the grown-ups; the child just needs the pattern, and it works for every number ending in 5.

➗ Division

18. Divide by 5

Double the number, then divide by 10.

Example: 145 ÷ 5 = 29

  1. Double it. 145 × 2 = 290
  2. Divide by 10, just drop the zero. 290 ÷ 10 = 29

Why it works: dividing by 5 is the same as dividing by 10 and doubling, because 5 = 10 ÷ 2. Doubling first keeps everything in whole numbers.

19. Half-Half (÷4)

Divide by 4 by halving twice.

Example: 112 ÷ 4 = 28

  1. Halve it. 112 ÷ 2 = 56
  2. Halve it again. 56 ÷ 2 = 28

Why it works: 4 = 2 × 2, so dividing by 4 is halving twice. Halving is easier than dividing, even for large numbers.

20. Chunking

Split a big number into two friendly chunks and divide each.

Example: 132 ÷ 6 = 22

  1. Split 132 into 120 and 12. Divide the big chunk. 120 ÷ 6 = 20
  2. Now the small chunk. 12 ÷ 6 = 2
  3. Add the two answers. 20 + 2 = 22

Why it works: division shares out a total, and the total can be split into any convenient pieces first. Choose a big chunk that is an easy multiple of the divisor, then handle the small remainder.

Which tricks to teach first

Start with the ones that lean on tens: making tens, plus 9, minus 9, and friendly numbers. They build the single most useful idea in mental arithmetic, that ten is the place to stand. Doubles, times 4 and times 5 come next, because doubling and halving feel like a game. Leave the 11 rule and squaring numbers ending in 5 for last; they are the party tricks, and they land better once a child trusts the others.

Math Ninja teaches each trick three ways: watch a worked example, do one together with each step filled in, then five on your own. Stars are saved per trick, so a child can see the twenty light up over a few weeks.

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