The Speedy 11x Multiplier!

Ever wish you could multiply ANY number by 11 in seconds, without a calculator? Learn this lightning-fast trick!

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

The Trick

Here's the magic for $N \times 11$: 1. **For 2-digit numbers (e.g., $AB$):** Take the number, separate its digits. Add the two digits together. Place this sum between the original two digits. If the sum is $\ge 10$, carry over the '1' to the first digit. 2. **For multi-digit numbers (e.g., $ABCDE$):** Place the first digit ($A$) at the start and the last digit ($E$) at the end. For the digits in between, start from the right and add adjacent pairs: $(E+D)$, then $(D+C)$, then $(C+B)$, then $(B+A)$. Write each sum in order. Remember to carry over any '1' if a sum is $\ge 10$. **Why it works:** Multiplying $N$ by 11 is equivalent to $N \times (10+1) = (N \times 10) + N$. This trick cleverly performs that addition by shifting digits and summing them up visually/mentally.

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