The Super-Speedy '11x' Trick!

Ever wish you could multiply numbers by 11 in your head, almost instantly? Get ready to impress your friends and ace your calculations!

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

The Trick

To multiply any two-digit number by 11, imagine a space between its two digits. Add the two digits together and place their sum in that space! If the sum is $10$ or more, write down the units digit of the sum and carry over the tens digit to the first digit. This works due to the distributive property: $N \times 11 = N \times (10+1) = (N \times 10) + (N \times 1)$. For a number 'ab', this is equivalent to $100a + 10(a+b) + b$, which directly shows the pattern of the trick!

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