The Speedy 11x Multiplier!

Ever wish you could multiply ANY number by 11 in mere seconds, without a calculator? Get ready to amaze yourself!

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

The Trick

To multiply any number by $11$: 1. Write the first digit of the number. 2. Write the last digit of the number. 3. In between, sum adjacent digits from right to left. If a sum is $\ge 10$, carry over the tens digit to the left. WHY? This works because $N \times 11 = N \times (10+1) = 10N + N$. You're essentially shifting the number left and adding it to itself, which aligns the place values for this 'sandwich' effect.

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