How to Multiply 2 Digit Numbers by 9 Really Fast

We have had several posts regarding lightning speed multiplication and in this post, we are going to learn another trick on how to multiply by 9. Note that this method is only possible if the ones digit is greater than the tens digit.

How to Multiply 2 Digit Numbers by 9 Fast

To get the product of 2 digit number multiplied by 9

1.) Copy the tens digit of the 2-digit number as the hundreds digit of the product.

2.) Count the numbers between the the tens and ones digit of the 2-digit number and make it the tens digit of the product.

3.) Count how many numbers from the ones digit of the 2-digit number up to 10 including 10, and then make it the ones digit of the product. 

Example 1: 58 × 9

1.) We copy the tens digit of the 2-digit number which is 5. This is the number on the hundreds place of the product.
Hundreds digit of the product: 5

2.) Count how many numbers between 5 and 8. There are 2 (6 and 7). So, the tens digit of the product is 2.
Tens digit of the product: 2

3.) Next, how many numbers after the ones digit up to 10 including 10? There are 2 (9 and 10).
Ones digit of the product: 2

So, 58× 9 = 522.

Example 2: 47 × 9

1.) Copy 4 as hundreds digit of the product.

2.) There are 2 numbers between 4 and 7 (5 and 6), so the tens digit is 2.

3.) There are 3 numbers up to 10 after 7 up to 10 including 10. So the ones digit is 3.

So, 47× 9 = 423.

Example 3: 89 × 9

1.) Copy 8 as hundreds digit of the product.
2.) There are no numbers between 8 and 9 so the tens digit of the product is 0.
3.) How many numbers after 9 up to 10 including 10. There’s 1.

So, 89× 9 = 801.

There’s our math trick. It’s easy and really fast. Share it to your friends!

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