The Speedy 11x Multiplier

Ever wish you could multiply ANY two-digit number by 11 in seconds, without a calculator? Get ready to amaze yourself and your friends!

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

The Trick

To multiply a two-digit number ($AB$) by 11: 1. Write digit $A$ as the first digit of your answer. 2. Write digit $B$ as the last digit of your answer. 3. Add the two digits together: $A+B$. 4. Place the sum $A+B$ in between $A$ and $B$. If $A+B$ is a two-digit number, carry over the tens digit to $A$. **WHY it works:** Multiplying $AB$ by $11$ is like $AB \times (10+1) = (10A+B) \times 10 + (10A+B) \times 1 = 100A + 10B + 10A + B = 100A + 10(A+B) + B$. This reveals $A$ in the hundreds place, $A+B$ in the tens place, and $B$ in the units place!

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