The Speedy 11x Multiplier

Ever wished you could multiply any two-digit number by 11 in seconds, without a calculator? Get ready to amaze yourself with this simple trick!

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

The Trick

Here's how to multiply any two-digit number by 11 super fast! For a number, say $AB$ (where $A$ is the tens digit and $B$ is the units digit), just sum the two digits ($A+B$) and place that sum right in between $A$ and $B$. If the sum is 10 or more, carry over the 'tens' digit of the sum to the 'A' digit. This works because multiplying by $11$ is like multiplying by $(10+1)$, so $AB \times 11 = AB \times 10 + AB \times 1 = ABO + AB$, which cleverly aligns the addition of digits in a visual way!

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