The Fast Five Multiplier!

Tired of long multiplication? What if you could multiply ANY number by 5 in a blink? Get ready to supercharge your mental math skills!

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

The Trick

To multiply any number ($N$) by 5, use this two-step trick: 1. **If $N$ is EVEN:** Divide $N$ by 2, then append a '0' to the result. 2. **If $N$ is ODD:** Subtract 1 from $N$ (making it even), divide that by 2, then append a '5' to the result. **Why it works:** $5$ is the same as $\frac{10}{2}$. So, $N \times 5 = N \times \frac{10}{2} = \frac{N}{2} \times 10$. If $N$ is even, $N/2$ is an integer, so multiplying by 10 is just adding a zero. If $N$ is odd, $(N-1)$ is even, so $(N-1)/2$ is an integer. $N \times 5 = ((N-1) + 1) \times 5 = (N-1) \times 5 + 1 \times 5 = \frac{N-1}{2} \times 10 + 5$. This is equivalent to appending a 5!

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