Learning Objectives
- Learn quick tricks to multiply by 10, 100, and 5 in your head.
- Use shortcuts to add or subtract 9 and 99 quickly.
- Practise doubling, halving, and adding near-100 numbers mentally.
- Solve 30+ rapid-fire problems using mental maths tricks.
Let Us Begin!
Mental maths means doing calculations in your head -- without writing anything down. It is like a superpower for numbers. The more you practise, the faster you get.
Today you will learn simple tricks that will help you solve problems quickly. Ready? Let us go!
[Image: A thought bubble showing mental calculation shortcuts: "25 × 4 = 100", "To add 9, add 10 then subtract 1", "Double 35 = 70". Title: "Mental Maths Tricks"]
Trick 1: Multiply by 10 -- Just Add a Zero
To multiply any number by 10, simply add one zero at the end.
Examples:
7 x 10 = 70 | 23 x 10 = 230 | 156 x 10 = 1560
Trick 2: Multiply by 100 -- Add Two Zeros
To multiply any number by 100, add two zeros at the end.
Examples:
5 x 100 = 500 | 34 x 100 = 3400 | 120 x 100 = 12000
Trick 3: Multiply by 5 -- Multiply by 10, Then Halve
Multiplying by 5 is easy: first multiply by 10 (add a zero), then divide by 2 (take half).
Examples:
14 x 5: Step 1: 14 x 10 = 140. Step 2: 140 / 2 = 70.
36 x 5: Step 1: 36 x 10 = 360. Step 2: 360 / 2 = 180.
48 x 5: Step 1: 48 x 10 = 480. Step 2: 480 / 2 = 240.
Trick 4: Add 9 -- Add 10, Then Subtract 1
Adding 9 is the same as adding 10 and then taking away 1.
Examples:
45 + 9: 45 + 10 = 55, then 55 - 1 = 54.
78 + 9: 78 + 10 = 88, then 88 - 1 = 87.
Trick 5: Subtract 9 -- Subtract 10, Then Add 1
Subtracting 9 is the same as subtracting 10 and then adding 1 back.
Examples:
63 - 9: 63 - 10 = 53, then 53 + 1 = 54.
82 - 9: 82 - 10 = 72, then 72 + 1 = 73.
Trick 6: Doubling and Halving
Doubling means multiplying by 2. Halving means dividing by 2. These are useful building blocks.
Examples:
Double 35 = 70 | Double 120 = 240 | Double 450 = 900
Half of 64 = 32 | Half of 150 = 75 | Half of 500 = 250
Tip: To multiply by 4, double twice. Example: 13 x 4 = double 13 = 26, double 26 = 52.
Trick 7: Adding Near-100 Numbers
To add 99, add 100 and subtract 1. To add 98, add 100 and subtract 2.
Examples:
250 + 99: 250 + 100 = 350, then 350 - 1 = 349.
175 + 98: 175 + 100 = 275, then 275 - 2 = 273.
460 + 97: 460 + 100 = 560, then 560 - 3 = 557.
Key Words
| Word | Meaning |
|---|
| Mental Maths | Doing calculations in your head without writing them down. |
| Doubling | Multiplying a number by 2. |
| Halving | Dividing a number by 2. |
| Shortcut | A quicker way to solve a problem. |
⭐ Key Points to Remember
- Multiply by 10: add one zero. Multiply by 100: add two zeros.
- Multiply by 5: multiply by 10, then halve.
- Add 9: add 10, subtract 1. Subtract 9: subtract 10, add 1.
- Add 99: add 100, subtract 1. Add 98: add 100, subtract 2.
- Double = multiply by 2. Halve = divide by 2.
- Multiply by 4: double twice. Multiply by 20: multiply by 10, then double.
Practice Questions
A. Multiply by 10 and 100
1. 15 x 10 =
2. 42 x 10 =
3. 230 x 10 =
4. 8 x 100 =
5. 56 x 100 =
B. Multiply by 5
1. 12 x 5 =
2. 24 x 5 =
3. 18 x 5 =
4. 32 x 5 =
5. 50 x 5 =
C. Add and Subtract 9
1. 36 + 9 =
2. 54 + 9 =
3. 91 - 9 =
4. 70 - 9 =
5. 125 + 9 =
D. Doubling and Halving
1. Double 45 =
2. Double 125 =
3. Half of 90 =
4. Half of 260 =
5. Double 350 =
E. Adding Near-100 Numbers
1. 150 + 99 =
2. 225 + 99 =
3. 340 + 98 =
4. 180 + 97 =
5. 500 + 99 =
F. Mixed Rapid Fire -- Use Any Trick!
1. 19 x 10 =
2. 16 x 5 =
3. 88 + 9 =
4. Half of 180 =
5. 67 - 9 =
6. 44 x 5 =
7. Double 75 =
8. 300 + 99 =
9. 13 x 100 =
10. Half of 420 =
Speed Challenge -- Beat the Clock!
Ask a friend or parent to time you. Try to solve all 10 problems in under 2 minutes. Write only the answers.
| No. | Problem | Answer |
|---|
| 1 | 25 x 10 | |
| 2 | 14 x 5 | |
| 3 | 56 + 9 | |
| 4 | 100 - 9 | |
| 5 | Double 65 | |
| 6 | Half of 300 | |
| 7 | 450 + 99 | |
| 8 | 38 x 5 | |
| 9 | 72 - 9 | |
| 10 | 11 x 100 | |
My time: ____________ minutes. I got ________ out of 10 correct.