Lightning Squares: Numbers Ending in 5!

Ever wish you could square big numbers like $45$ or $85$ instantly? Unlock a secret trick that makes it feel like magic!

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

The Trick

To square any number ending in $5$: 1. Take the 'tens' digit (or the number before the $5$). 2. Multiply this digit by the next consecutive whole number. 3. Append '25' to the result. **Why it works:** Let the number be $(10x+5)$. Squaring it gives $(10x+5)^2 = (10x)^2 + 2(10x)(5) + 5^2 = 100x^2 + 100x + 25 = 100x(x+1) + 25$. The $x(x+1)$ part is your first step, and the $100$ shifts it to the hundreds place, effectively letting you append $25$.

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