The Nines Multiplier Magic

Ever wish you could multiply numbers by 9, 99, or 999 super fast? What if you could do it mentally, in seconds, without a calculator?

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

The Trick

Here's the secret: $N \times 9$ is just $N \times (10-1)$. So, it becomes $10N - N$. Similarly, $N \times 99$ is $100N - N$, and $N \times 999$ is $1000N - N$. You multiply by a power of 10 (which is easy!) and then subtract the original number. This works because numbers like 9, 99, 999 are exactly one less than a power of 10 ($10^1-1, 10^2-1, 10^3-1$). It simplifies multiplication into a much easier subtraction problem.

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