The Speedy 11x Multiplier!

Tired of long multiplication? What if you could multiply ANY two-digit number by 11 in seconds, WITHOUT a calculator?

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

The Trick

To multiply any two-digit number (let's say $AB$) by 11: 1. Separate the two digits: $A \_ B$. 2. Add the two digits together: $A+B$. 3. Place this sum in the middle: $A (A+B) B$. 4. If $A+B$ is a two-digit sum (e.g., 14), write the units digit in the middle and carry over the '1' to the 'A' digit. Why it works: $11 \times (10A + B) = 110A + 11B = 100A + 10A + 10B + B = 100A + 10(A+B) + B$. This directly translates to $A$ in the hundreds place, $A+B$ in the tens, and $B$ in the units.

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