The Speedy 11x Multiplier Trick!

Ever wish you could multiply ANY number by 11 in seconds, without a calculator? Get ready to amaze yourself and your friends with this incredible mental math shortcut!

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

The Trick

To multiply any number by 11, follow these steps: 1. Write down the rightmost digit of the original number. 2. Moving left, add each digit to its neighbor on the right. Write down the sum. If the sum is a two-digit number, carry over the tens digit to the next addition. 3. Finally, write down the leftmost digit of the original number (adding any carry-over from the previous step). Why it works: Multiplying by 11 is the same as multiplying by $(10+1)$. So, $N \times 11 = (N \times 10) + (N \times 1)$. When you add $N$ to $N \times 10$ (which is $N$ with a zero at the end), you're essentially summing digits based on their place value.

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