Reaching for your phone every time you need to calculate a tip, split a bill, or check a discount? You don’t have to. These 10 tricks are the same building blocks we teach in our abacus and mental math classes at Mission Abacus — and every one of them can be learned in a few minutes and mastered with a little daily practice.

Try a few of these on your next shopping trip. Once they click, you’ll wonder why you ever reached for a calculator.

1. Multiply by 9 Using the “Minus One” Trick

Multiplying by 9 feels hard until you realize 9 is just 10 minus 1. So instead of multiplying by 9, multiply by 10 and subtract the original number.

Example: 8 × 9 → (8 × 10) − 8 = 80 − 8 = 72

This one trick alone makes the entire 9-times table effortless.

2. Multiply Any Two-Digit Number by 11

This is the trick that gets the biggest “wait, what?!” reaction. Split the two digits apart, add them together, and drop the sum in the middle.

Example: 11 × 43 → 4 _ 3 → 4+3 = 7 → 473

(If the digits add up to more than 9, you carry the 1 — but for most everyday numbers, this works instantly.)

3. Square Any Number Ending in 5

Take the digit(s) before the 5, multiply that number by itself plus one, then stick 25 on the end.

Example: 35² → 3 × (3+1) = 3 × 4 = 12 → 1225

Try it on 45² or 65² — it works every single time.

4. Round, Then Adjust

For messy multiplication, round one number to the nearest 10 first, then correct the result.

Example: 48 × 5 → round to 50 × 5 = 250 → subtract (2 × 5) = 250 − 10 = 240

This turns awkward multiplication into simple mental steps.

5. Break Numbers Into Friendly Parts

Instead of adding large numbers directly, split them into rounder pieces.

Example: 47 + 38 → (47 + 30) + 8 = 77 + 8 = 85

This is one of the very first techniques we teach young students because it builds real number sense, not just memorized shortcuts.

6. Use “Pairs to 10” for Lightning-Fast Addition

Knowing which pairs of digits add up to 10 (1+9, 2+8, 3+7, 4+6) makes single-digit addition almost automatic.

Example: 6 + 7 → think 6 + 4 = 10, then add the leftover 3 → 13

Kids who master this early tend to add multi-digit numbers noticeably faster later on.

7. Multiply by 5 the Easy Way

Instead of multiplying by 5 directly, multiply by 10 and cut the result in half.

Example: 48 × 5 → 48 × 10 = 480 → 480 ÷ 2 = 240

Works instantly for any number, no rounding required.

8. Divide by 5 by Doubling First

To divide by 5, double the number, then divide by 10.

Example: 180 ÷ 5 → 180 × 2 = 360 → 360 ÷ 10 = 36

9. Find 15% in Two Quick Steps

Instead of calculating an odd percentage directly, find 10% first (move the decimal point), then add half of that for the extra 5%.

Example: 15% of 80 → 10% = 8 → 5% = 4 → 8 + 4 = 12

This is the exact trick that makes calculating tips at a restaurant almost instant.

10. Cross-Multiply for Two-Digit Multiplication

Break both numbers into tens and units, then multiply each part separately before adding them together.

Example: 23 × 14 → (20+3) × (10+4) = 200 + 80 + 30 + 12 = 322

This looks intimidating written out, but with practice it becomes a smooth, automatic process — which is exactly what abacus training accelerates.

Why These Tricks Work Better With Abacus Training

Here’s the honest truth: these tricks are useful shortcuts, but they’re memorized patterns for specific number types. Abacus training teaches something different — an underlying mental model for all numbers, so calculation speed isn’t limited to a handful of memorized cases.

Students who go through structured abacus and mental math training tend to pick up these kinds of tricks faster too, because they’ve already built strong number sense and working memory through consistent practice.

Try This Today

Pick just one trick from this list and use it three times today — while shopping, checking a bill, or helping with homework. Repetition is what turns a “trick” into an instinct.

Want your child to build this kind of number fluency from the ground up? [Book a free trial class with Mission Abacus] and see how structured mental math training compares to memorized shortcuts.

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About the Author: Written by the Mission Abacus teaching team, drawing on classroom techniques used to build calculation speed and number sense in students of all ages.

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FAQ (for FAQ Schema)

Q: Are mental math tricks the same as abacus training? A: Not quite. Mental math tricks are memorized shortcuts for specific types of calculations, while abacus training builds a general visual model for numbers that works across all calculation types.

Q: What’s the easiest mental math trick to start with? A: The “pairs to 10” trick (knowing 1+9, 2+8, 3+7, 4+6) is the easiest starting point and forms the foundation for faster addition and subtraction.

Q: Can adults learn these tricks, or is it just for kids? A: These tricks work at any age. Adults often pick them up even faster since they already have strong number sense — the challenge is usually just building the habit of using them.

Q: How long does it take to get fast at mental math? A: Individual tricks can be learned in minutes, but true speed and automaticity typically take a few weeks of regular, short practice sessions (5-10 minutes a day).