Kid Who Did presentsSHOW-OFF MATH!Become faster than a calculator

The math tricks

A printable book of 29 mental math tricks. Learn them yourself, or hand it to your kid and race each other to the answer.

Get the book for $27 See the tricks work
A kid races a calculator on 11 × 43 and says 473 while the calculator's display still shows 11.

A hands-on preview: five of the book's 29 tricks

Animated demo of High Five! 5 × 38 is worked by multiplying by 10 first, putting a 0 on the end to make 380, then cutting it in half to get 190. More examples show each answer is half the number with a 0 on the end: 5 × 16 = 80, 5 × 52 = 260 and 5 × 24 = 120. It ends with 5 × 64 to try.

Trick 8 High Five!

Multiply by 5 in your head.

Let's do 5 × 38

  1. Times 10 first: put a 0 on the end. 38 becomes 380.
  2. Cut it in half: half of 380 is 190.
Works for
any whole number
Animated demo of the 11 Sandwich. 11 × 43 is worked by splitting 43 into 4 and 3, adding them to get 7, and putting the 7 in the middle to make 473. More examples show the middle digit. In the carry case, 11 × 57, the digits add to 12: the last digit, 2, goes in the middle and the 5 becomes 6, making 627. It ends with 11 × 25 to try.

Trick 10 The 11 Sandwich

Times 11 for any two-digit number.

Let's do 11 × 43

  1. Split 43 into 4 and 3 (the bread).
  2. Add them to make the filling: 4 + 3 = 7.
  3. Put the filling in the middle: 473.
Works for
two-digit numbers whose digits add to 9 or less, like 43
Watch out
Digits add to 10 or more? Last digit in the middle, carry the 1: 11 × 57 = 627.
Animated demo of The Flip. 8% of 50 is flipped to 50% of 8, which is half of 8, which is 4. More examples follow, and a second worked example flips 12% of 25 to 25% of 12, a quarter of 12, which is 3. It ends with 6% of 50 to try.

Trick 19 The Flip

Find tricky percents by flipping the numbers around.

What's 8% of 50?

  1. Flip it: 8% of 50 is the same as 50% of 8.
  2. 50% means half. Half of 8 is 4.
Works for
any percent of any number
Flip when
The new percent is easy: 50% (half), 25% (a quarter) or 10%.
Animated demo of Tip Wizard. A $47.83 bill is rounded to $48, 10% is found by moving the point one place left to get $4.80, half of that is $2.40, and together they make about $7.20, the 15% tip. Doubling the 10% gives a 20% tip of about $9.60. More examples follow, and it ends with 15% of $40 to try.

Trick 18 Tip Wizard

Work out a tip for your server: 10%, 15% or 20%.

15% tip on a $47.83 bill

  1. Round the bill to the nearest dollar: $47.83 becomes $48.
  2. For 10%, move the point one place left: $4.80.
  3. For 15%, add half of that (5%): $4.80 + $2.40 is about $7.20.
  4. For 20%, double the 10%: about $9.60.
Works for
any bill (your tip comes out within 10¢)
Watch out
Bill ends in 50¢ or more? Round up: $35.75 → $36.
Animated demo of Five-Squared Magic. For 35 × 35, cover the 5, multiply 3 by the next number up, 4, to get 12, then write 25 on the end to make 1,225. More examples follow: 15 × 15, 25 × 25 and 45 × 45, each ending in 25. It ends with 75 × 75 to try.

Trick 22 Five-Squared Magic

Multiply a whole number that ends in 5 by itself, fast.

Let's do 35 × 35

  1. Cover the 5. Times what's left (3) by the next number up (4): 3 × 4 = 12.
  2. Write 25 on the end: 35 × 35 = 1,225.
Works for
whole numbers ending in 5, like 15, 35 or 105
Even bigger
Cover the 5 in 105 and 10 is left: 10 × 11 = 110, then 25: 11,025.

29 tricks like these, plus a finale magic show.

Get the book for $27

Each trick gets a lesson, a game and a challenge

The How To Do It box from the 11 Sandwich lesson page: numbered steps, a picture of the sandwich, and the worked answer.

1 Learn it

The lesson page shows the trick on a real example, then a second one, then three problems to try, with the answers printed upside down at the bottom.

Part of the Tip Wizard activity page: the rule card, the how-to-play lines, and a restaurant receipt with the first tip already filled in.

2 Play it

The next page turns the trick into a game: secret codes, mazes, bingo, restaurant receipts. Many pages start with the first problem already worked in pencil.

The SHOW OFF strip at the bottom of an activity page, with a challenge for a grown-up and a line for who your kid showed it to.

3 Show it off

Each activity page ends with a SHOW OFF challenge to spring on someone else, and a line to write down who got stumped.

Six levels, from warm-ups to a magic show

Level 1 is quick wins with small numbers. The tricks get bigger from there, and Level 6 is a math magic show to perform for the family.

  1. 1 Warm-ups

    Quick wins with small numbers. Start here!

    • Ten Buddies
    • The Sneaky 9
    • Double Trouble
    • Jump to 10
    • Back to 10

  2. 2 Times-table cheats

    Multiply without memorizing every single fact.

    • Zero Hero
    • Double, Double!
    • High Five!
    • The Nines Hands
    • The 11 Sandwich

  3. 3 Big number moves

    Break big numbers into easy pieces.

    • The Big Nines
    • Big Ones First
    • Slide It!
    • Snap It Apart
    • Split to Divide

  4. 4 Money brain

    Tricks for stores, restaurants and big sales.

    • Round It, Fix It
    • Make Change
    • Tip Wizard
    • The Flip
    • Slice It

  5. 5 Wow tricks

    The ones that make grown-ups say "Wait, HOW?"

    • Half and Double
    • Five-Squared Magic
    • The Teen Machine
    • Extra Filling
    • Divisibility Detective

  6. 6 The big show

    Math magic to perform for an audience.

    • The Mind Reader
    • The 1089 Prediction
    • The Number Bender
    • Calendar Speed-Add
    • The Big Show

Open it to a random page

Each page opens with what the trick does and the rules for using it, so there's no flipping back to find out what a page wants.

The lesson page for Trick 10, The 11 Sandwich, with steps, a worked example, the carry case, three problems, why it works, and a note for grown-ups.
A lesson page (Trick 10): the steps on a real example, the carry case, three problems with upside-down answers, why it works, and a note for grown-ups.
The activity page for Trick 18, Tip Wizard, with four restaurant receipts to work out tips on.
An activity page (Trick 18): four restaurant receipts, with the first tip already worked in pencil.
The activity page for Trick 27, the 1089 Prediction, with a prediction slip to cut out, an audience card, and a practice table.
A magic kit (Trick 27): cut out the prediction slip and the audience card, then practice on three numbers.

For you, or for your kid

If it's for you

  • Each lesson has a WHY IT WORKS box in plain words, so you know why the shortcut gives the right answer.
  • Level 4 is everyday money math: tips, percents, sale prices, making change and prices like $1.99.
  • Level 6 is a set of math magic tricks to perform at the dinner table.
  • Level 1 is warm-ups. Skip ahead if they feel easy.

If it's for your kid

  • The deal for your kid: learn a trick, stump a grown-up. Each activity page ends with a SHOW OFF challenge for that.
  • The answer key is in the back, so you can check a page without working it first.
  • Where a trick has a school name (number bonds, compensation, partial quotients), the lesson's note for grown-ups gives it, so homework help stays easy.
  • There are no timed tests. The calculator races are games your kid chooses to play.
  • The book is printed on paper, so it's screen-free.
The cover of Show-Off Math!, with 11 × 43 = 473! and Beep the calculator.

What you get for $27

  • The 73-page PDF: US Letter, black and white, ready to print.
  • 29 trick lessons and a finale show, each with its own activity page.
  • A SHOW OFF challenge on each activity page, and a certificate at the end.
  • A three-page answer key.
  • The Math Magic Show kit: a program, tickets and an applause meter.
Get the book for $27

One payment. The PDF downloads right after checkout.

The 30-Day Show-Off Guarantee

Try the tricks for 30 days. If you (or your kid) haven't stumped somebody with one by then, let us know and we'll refund the $27.

Questions before you buy

Who is it for?

Kids 8 to 12 (US grades 3 to 6), and the grown-ups who want to keep up, as the cover puts it. Level 1 is easy enough for some 7-year-olds, and older kids can start at Level 2.

Can my kid use it without me?

That's how the pages are written. Each one carries its own instructions and rules, so there's nothing to look up. A few games need a second person: the calculator races, bingo and the magic tricks. The grown-up note on each lesson and the answer key at the back are for you.

Are these tricks to memorize, or does the book explain why they work?

Each lesson has a WHY IT WORKS box in plain words, and the grown-up note names the school method where there is one. The goal is a shortcut you can explain, not just run.

Will my kid be racing a clock?

No. The book has no timed tests. The calculator races are a game: a grown-up types, your kid says the answer out loud, and your kid decides whether to play.

What do I need to print it?

A home printer and US Letter paper, in black and white. A pencil does the work on the lesson pages. A few activities use scissors, and the calculator races use a calculator.

Try it yourself first, or hand your kid a pencil and Trick 1 and see how far they get on their own.

Get the book for $27
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Terms

Last updated October 27, 2026

These terms cover buying and using Show-Off Math!, a printable book of mental math tricks from Kid Who Did, sold as a download. "We" and "us" means Kid Who Did. "You" means the person who buys it. Buying the book means you agree to these terms.

What you're buying

A 73-page PDF, sized for US Letter paper, that you download after checkout. Nothing is shipped. The price is in US dollars and you pay it once: there's no subscription and nothing renews.

Printing and sharing

The tricks are yours to use and teach. The book's pages, drawings and words remain ours.

The 30-day Show-Off guarantee

Try the tricks for 30 days. If you (or your kid) haven't stumped somebody with one by then, reply to your receipt email within 30 days of buying and we'll refund your $27. We'll take your word for it, and you don't need to send anything back.

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Checkout and payment are handled by our payment provider, so your card details go to them, not to us. After you pay, you get your download link on the confirmation page and in your receipt email. If the link doesn't work, reply to that email and we'll send a new one.

Kids and safety

You need to be 18 or older to buy the book. Kids use it with your say-so. A few activities use scissors, so younger kids should have a grown-up nearby for those.

What the book is, and isn't

Show-Off Math! is for learning and for fun. It isn't tutoring, and it doesn't promise grades or test scores. We work hard to get each trick and answer right, and the book is provided as it is. If something goes wrong, our responsibility to you is limited to what you paid for the book.

Changes

We may update these terms, and the date at the top shows the latest version. The terms in place when you bought still cover your purchase.

The law that applies

These terms are governed by the laws of the State of Arizona, USA.

Questions

Reply to your receipt email and we'll get back to you.
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Privacy

Last updated October 27, 2026

This policy covers Show-Off Math! and this page, from Kid Who Did ("we" and "us"). The short version: we collect what we need to sell you the book and help you with it, and this page uses two tools, from Meta and Microsoft, that show us how our ads and the page are working. We don't sell or rent our customer list.

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