Want to multiply by 89 faster? Use the fact that 89 is just 1 less than 90. Instead of multiplying by 89 directly, multiply by 90 and subtract the original number. This simple method makes the 89 times table much easier to calculate mentally.
The shortcut works for every whole number. For example, 7 × 89 becomes 7 × 90 − 7, which equals 630 − 7 = 623.
The Quick Formula
The reason is simple:
Using the distributive property:
Example: 7 × 89
Let us use the trick step by step:
The 89 Times Table
Here is the complete multiplication table from 1 through 10:
| Multiplication | Using 90 − 1 | Answer |
|---|---|---|
| 1 × 89 | 90 − 1 | 89 |
| 2 × 89 | 180 − 2 | 178 |
| 3 × 89 | 270 − 3 | 267 |
| 4 × 89 | 360 − 4 | 356 |
| 5 × 89 | 450 − 5 | 445 |
| 6 × 89 | 540 − 6 | 534 |
| 7 × 89 | 630 − 7 | 623 |
| 8 × 89 | 720 − 8 | 712 |
| 9 × 89 | 810 − 9 | 801 |
| 10 × 89 | 900 − 10 | 890 |
Why This Trick Works
The shortcut is an application of the distributive property. Since 89 can be written as 90 − 1, multiplying by 89 means multiplying by 90 and removing one copy of the original number.
For example, with 12:
You can check the answer directly: 1,068 ÷ 89 = 12.
A Second Method: Use 100 − 11
Another useful way to rewrite 89 is 100 − 11:
This can be helpful when multiplying by larger numbers:
Both methods are correct. Choose the form that makes the mental calculation easiest.
Spot the Pattern in the Answers
The answers in the 89 times table are:
Each answer increases by 89. The final digits also follow a repeating pattern: 9, 8, 7, 6, 5, 4, 3, 2, 1, 0. This pattern can help you check your work, but the formula 90n − n is the reliable calculation method.
More Examples
Common Mistakes to Avoid
Do not subtract 1 from the final answer after multiplying by 90. The correct rule is to subtract the original multiplier. For 7 × 89, calculate 7 × 90 = 630, then subtract 7—not 1.
Do not confuse 89 with 90. The shortcut works because 89 is one less than 90.
Remember the order of operations in the expanded expression: multiply the number by 90, then subtract the same number.
Practice Problems
Use the rule n × 89 = 90n − n to solve these:
- 4 × 89
- 11 × 89
- 18 × 89
- 24 × 89
- 35 × 89
Answer Key
- 4 × 89 = 360 − 4 = 356
- 11 × 89 = 990 − 11 = 979
- 18 × 89 = 1,620 − 18 = 1,602
- 24 × 89 = 2,160 − 24 = 2,136
- 35 × 89 = 3,150 − 35 = 3,115
Final Takeaway
To multiply by 89 quickly, rewrite 89 as 90 − 1:
For the video’s examples, this gives the complete pattern from 89 to 890. Once you learn this shortcut, multiplying by 89 becomes a fast, accurate mental-math skill.
Frequently Asked Questions
What is the easiest way to multiply by 89?
Multiply the number by 90, then subtract the original number. For example, 8 × 89 = 8 × 90 − 8 = 720 − 8 = 712.
What is 9 × 89?
9 × 89 = 810 − 9 = 801.
Why does the shortcut work?
It works because 89 = 90 − 1. The distributive property gives n × 89 = n × 90 − n.
What is 10 × 89?
10 × 89 = 890.
Can I use this trick for larger numbers?
Yes. The rule works for any whole number. For example, 125 × 89 = 125 × 90 − 125 = 11,250 − 125 = 11,125.
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