Speedy Squares for Numbers Ending in 5! 🚀

Ever wish you could square numbers like 35, 75, or even 105 in your head almost instantly? There's a fun trick that makes it super easy!

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

The Trick

To square any number ending in 5, like $N5$: 1. The last two digits of your answer will ALWAYS be $25$. 2. Take the digit(s) before the 5 (let's call it $N$). 3. Multiply $N$ by $(N+1)$. 4. Place this product in front of $25$. Why it works: A number ending in 5 is $(10N+5)$. Squaring it gives $(10N+5)^2 = (10N)^2 + 2(10N)(5) + 5^2 = 100N^2 + 100N + 25 = 100(N^2+N) + 25 = 100(N(N+1)) + 25$. This clearly shows the $N(N+1)$ part followed by $25$.

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