The Speedy 11x Multiplier

Tired of long multiplication? What if you could multiply ANY two-digit number by 11 in seconds, without a calculator? Get ready for a mind-blowing shortcut!

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

The Trick

Here's the trick: Take any two-digit number, say $AB$. To multiply it by 11, first separate the digits $A$ and $B$. Then, add the digits together: $A+B$. Now, place this sum in between $A$ and $B$. If the sum $A+B$ is a two-digit number, carry over the tens digit to $A$. **Why it works:** Let the number be $10A + B$. Multiplying by 11 gives $11(10A + B) = 110A + 11B = 100A + 10A + 10B + B = 100A + 10(A+B) + B$. This clearly shows $A$ in the hundreds place, $(A+B)$ in the tens place, and $B$ in the units place.

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