Multiplying by 11 Trick

The 11's trick is a simplified version of the “moving down the line” multiplication method. To multiply a number by 11, you add consecutive digits of the number and record the result, keeping track of any carries.

For example, to calculate $523 \times 11$:

• Ones digit: 3
• Tens digit: 2 + 3 = 5
• Hundreds digit: 5 + 2 = 7
• Thousands digit: 5

The result is 5753. This trick can be extended to multiplication by 111, where you add triples of digits.