Multiplication of a double digit number by 9. Case #1

In this page, we are going to discuss some tricks to multiply a double digit number by 9. We divide this into a few cases:

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 one, where the number is a multiple of 11.

Condition:

1. The multiplicand is a double digit number and also a multiple of 11.
2. The multiplier is 9.

Example 1

44 x 9 = ?

1. Let's say the answer is x. x must be a 3 digits number. The digit in the middle must be 9.
   
   
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

22 x 9 = ?

1. 22 x 9 = __ 9 __
2. 9 x 2 = 18

Therefore, 22 x 9 = 198

Example 3

88 x 9 = ?

1. 88 x 9 = __ 9 __
2. 9 x 8 = 72

Therefore, 88 x 9 = 792