Mental Math Tricks Flashcards
Free mental math flashcards with shortcuts for adding, subtracting, multiplying, dividing, squaring numbers, finding percents and estimating. Flip each card, try the trick on the example and drill the ones you miss.
Keys: Space or Enter flips, arrow keys move, K marks known, R marks review. No sign-up and nothing is stored.
How to study with these mental math tricks flashcards
- Read the front of the card and say or write your answer before you flip it.
- Flip the card, then mark I know this only if you were right without hesitating.
- Mark Review again for anything you missed, then use Review missed only to drill those cards.
- Pick one topic at a time, then shuffle all topics for a mixed review.
All 29 cards at a glance
Every card is also listed below so you can scan, print or review without the viewer.
Addition and subtraction
| Question | Answer |
|---|---|
| How do you add 9 or 99 quickly? | Add 10 (or 100), then subtract 1. 47 + 9 = 47 + 10 − 1 = 56. |
| How do you subtract 9 quickly? | Subtract 10, then add 1. 63 − 9 = 63 − 10 + 1 = 54. |
| What is the compensation strategy? | Round one number to a friendly number, then adjust. 68 + 27 = 70 + 27 − 2 = 95. |
| How do you add left to right? | Add the larger place values first. 47 + 38: 40 + 30 = 70, 7 + 8 = 15, total 85. |
| How do you subtract by counting up? | Count from the smaller number up to the larger. 100 − 63: 63 + 7 = 70, then 30 more is 37. |
| How do you add numbers that make 10 or 100? | Group them. 38 + 62 = 100 because the ones make 10 and the tens make 90. |
Multiplication and division
| Question | Answer |
|---|---|
| How do you multiply by 5? | Multiply by 10 and halve. 46 × 5 = 460 ÷ 2 = 230. |
| How do you multiply by 25? | Multiply by 100 and divide by 4. 36 × 25 = 3600 ÷ 4 = 900. |
| How do you multiply by 11? | Add the digits and place the sum between them. 36 × 11 = 3(3+6)6 = 396. |
| How do you multiply by 9? | Multiply by 10 and subtract the number. 7 × 9 = 70 − 7 = 63. |
| How do you double and halve to multiply? | Halve one factor and double the other. 16 × 35 = 8 × 70 = 560. |
| How do you use the distributive property? | Split one factor. 7 × 14 = 7 × 10 + 7 × 4 = 70 + 28 = 98. |
| How do you divide by 5? | Double the number and divide by 10. 70 ÷ 5 = 140 ÷ 10 = 14. |
| How do you divide by 4? | Halve twice. 52 ÷ 4: 52 → 26 → 13. |
Squares and special patterns
| Question | Answer |
|---|---|
| How do you square a number ending in 5? | Multiply the tens digit by the next integer and write 25. 35²: 3 × 4 = 12, so 1225. |
| How do you use the difference of squares to multiply? | (a − b)(a + b) = a² − b². 29 × 31 = 30² − 1 = 899. |
| How do you square a number near 50? | Use (50 − d)² = 2500 − 100d + d². 48² = 2500 − 200 + 4 = 2304. |
| How do you square a number near 100? | (100 + d)² = 10000 + 200d + d². 103² = 10000 + 600 + 9 = 10609. |
| What is 15², 25² and 45²? | 225, 625 and 2025. |
| What are the first 10 perfect squares? | 1, 4, 9, 16, 25, 36, 49, 64, 81, 100. |
Percents and estimating
| Question | Answer |
|---|---|
| How do you find 10% of a number? | Move the decimal point one place left. 10% of 84 = 8.4. |
| How do you find 5%? | Half of 10%. 5% of 84 = 4.2. |
| How do you find 15%? | 10% plus 5%. 15% of 60 = 6 + 3 = 9. |
| How do you find 20%? | Double 10%. 20% of 45 = 9. |
| How do you find 25%? | Divide by 4. 25% of 120 = 30. |
| How do you find 1%? | Move the decimal two places left. 1% of 350 = 3.5. |
| How can you flip a percent? | x% of y equals y% of x. 4% of 50 = 50% of 4 = 2. |
| How do you estimate a product? | Round each factor to a friendly number. 48 × 21 ≈ 50 × 20 = 1000. |
| How do you estimate a sum? | Round each number, then add. 396 + 203 ≈ 400 + 200 = 600. |
Frequently asked questions
Why learn mental math?
It builds number sense, speeds up tests and helps you catch calculator errors.
How fast can I get better?
Short daily practice of a few tricks beats one long session. Use Review missed only to repeat the tricks you forget.
Sources
More free math flashcards
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