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:
- The number is a multiple of 11.
- The tens digit is less than the ones digit by one.
- The tens digit is less than the ones digit.
- 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:
- The multiplicand is a double digit number and also a multiple of 11.
- The multiplier is 9.
Example 1
44 x 9 = ?
- Let's say the answer is x. x must be a 3 digits number. The digit in the middle must be 9.
- 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.)
22 x 9 = ?
- 22 x 9 = __ 9 __
- 9 x 2 = 18
Therefore, 22 x 9 = 198
Example 3
88 x 9 = ?
- 88 x 9 = __ 9 __
- 9 x 8 = 72
Therefore, 88 x 9 = 792
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