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!

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

The Trick

To multiply any two-digit number $AB$ by 11, where $A$ is the tens digit and $B$ is the units digit: 1. Write $B$ as the units digit of your answer. 2. Add $A$ and $B$. Place this sum in the middle (tens digit). 3. Write $A$ as the hundreds digit. If the sum $A+B$ is 10 or more, carry over the '1' to $A$. WHY it works: $11 \times (10A + B) = 110A + 11B = 100A + 10A + 10B + B = 100A + 10(A+B) + B$. This directly shows the digits!

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