The Speedy 11x Mental Math Trick!

Tired of long multiplication? What if you could multiply ANY two-digit number by 11 in mere seconds, WITHOUT a calculator? Get ready to amaze yourself!

Subject: mental-calculation • Classes: 6–12 • Difficulty: basic

The Trick

Here's the trick for multiplying any two-digit number $AB$ by 11: 1. Write down the first digit (A). 2. Write down the sum of the two digits ($A+B$) in the middle. 3. Write down the second digit (B) at the end. 4. If the sum ($A+B$) is a two-digit number (e.g., 10, 11, etc.), carry over the tens digit of the sum to the first digit (A). Why it works: Multiplying $AB$ by $11$ is equivalent to $AB \times (10+1) = (A \times 10 + B) \times 10 + (A \times 10 + B) \times 1 = A \times 100 + B \times 10 + A \times 10 + B = A \times 100 + (A+B) \times 10 + B$. This structure perfectly matches our trick of placing $A$, then $(A+B)$, then $B$ in the respective place values, with carrying for sums $>9$.

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