The 11x Magic Multiplier

Ever wish you could multiply ANY number by 11 in seconds, without a calculator? Prepare for some mental magic!

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

The Trick

The 11x Magic Multiplier: For any number $N$: 1. Write down the first digit of $N$. 2. Write down the last digit of $N$. 3. Between them, sum adjacent digits of $N$ from right to left. 4. If a sum is $\ge 10$, write its units digit and carry over the tens digit to the next sum (or to the first digit if it's the leftmost sum). WHY it works: $N \times 11 = N \times (10+1) = N \times 10 + N \times 1$. This is adding $N$ to itself shifted one place value left, which is precisely what summing adjacent digits achieves with carrying.

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