The Speedy 11x Multiplier! ⚡

Ever wish you could multiply ANY two-digit number by 11 in seconds, without a calculator? Prepare to impress your friends and ace those mental math challenges!

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

The Trick

Here's the super simple 'sandwich' trick for $N \times 11$ where $N$ is a two-digit number $AB$: 1. Take your number, e.g., $27$. 2. Separate the digits: $2 \_ 7$. 3. Add the two digits: $2+7 = 9$. 4. Place the sum in the middle: $297$. Done! $27 \times 11 = 297$. What if the sum is 10 or more? E.g., $58 \times 11$: 1. Separate: $5 \_ 8$. 2. Add digits: $5+8 = 13$. 3. Place the '3' in the middle, and 'carry over' the '1' to the first digit ($5+1=6$). 4. Result: $638$. Done! $58 \times 11 = 638$. Why it works: A two-digit number $AB$ is $10A+B$. When multiplied by 11, it's $(10A+B) \times 11 = (10A+B) \times (10+1) = 100A + 10B + 10A + B = 100A + 10(A+B) + B$. This directly shows $A$ in hundreds, $(A+B)$ in tens, and $B$ in units place, handling carry-overs naturally!

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