Multiplication of a double digit number by 9. Case #2

There are a few tricks to multiply a double digit number by 9, according to different conditions:

1. The number is a multiple of 11.
2. The tens digit is less than the ones digit by one.
3. The tens digit is less than the ones digit.
4. The tens digit is greater than the ones digit.

In this page, we are going to discuss about case two, where the number a tens digit less than the ones digit by one.

Condition:

1. The multiplicand is a double digit number where the tens digit is less than the ones digit by one.
2. The multiplier is 9.

Example 1

34 x 9 = ?

1. Let's say the answer is x. x must be a 3 digits number. The digit in the middle must be 0.
   
   
2. Multiply the first digit of the multiplicand by 9. In this case, 4 x 9 = 36. The product will be the first and third digit of x (see image below.)
   
   

Example 2

23 x 9 = ?

1. 23 x 9 = __ 0 __
2. 9 x 3 = 27

Therefore, 23 x 9 = 207

Example 3

78 x 9 = ?

1. 78 x 9 = __ 0 __
2. 9 x 8 = 72

Therefore, 78 x 9 = 702