The Speedy 11x Multiplier

Tired of long multiplication? What if you could multiply ANY two-digit number by 11 in seconds, just by looking at it?

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

The Trick

To multiply any two-digit number, say $AB$, by 11: 1. Write down the first digit ($A$). 2. Add the two digits together ($A+B$). 3. Write down the second digit ($B$). If the sum ($A+B$) is a single digit, place it directly between $A$ and $B$. If the sum ($A+B$) is a two-digit number, write the units digit in the middle and carry over the tens digit to the first digit ($A$). Why it works: $11 \times (10A + B) = 110A + 11B = 100A + 10A + 10B + B = 100A + 10(A+B) + B$. This structure directly corresponds to our trick!

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