Speedy Squares: Numbers Ending in 5!

Ever wished you could square any number ending in 5 super fast, even without a calculator? Get ready to amaze yourself!

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

The Trick

To square any number ending in 5, follow these two simple steps: 1. Take the digit(s) before the 5. Let's call this 'n'. 2. Multiply 'n' by the next consecutive integer (n+1). 3. Append '25' to the result from step 2. WHY it works: Any number ending in 5 can be written as $(10n+5)$. Squaring this gives $(10n+5)^2 = 100n^2 + 100n + 25 = 100n(n+1) + 25$. The $n(n+1)$ part is what you calculate, and multiplying by 100 effectively places it before '25'.

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