The Speedy 11x Multiplier!

Ever wished you could multiply any two-digit number by 11 in seconds, without a calculator? Forget long multiplication! This trick will make you a math wizard!

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

The Trick

To multiply a two-digit number ($AB$) by 11, simply add the two digits ($A+B$). If the sum is less than 10, place it right between $A$ and $B$. If the sum is 10 or more, place the units digit of the sum between $A$ and $B$, and carry over the tens digit of the sum to $A$. \n\n**Why it works:** Let the number be $10A + B$. Then $(10A + B) \times 11 = (10A + B) \times (10 + 1) = 100A + 10B + 10A + B = 100A + 10(A+B) + B$. This shows $A$ in the hundreds place, $(A+B)$ in the tens place, and $B$ in the units place, handling carries correctly.

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