Brain Teasers & Mental Math
Mental math shortcut: square any two-digit number ending in 5, e.g. 35² and 65².
Model answer
For (10n + 5)²: take n × (n + 1), then append '25'.
35² → 3 × 4 = 12 → 1,225. 65² → 6 × 7 = 42 → 4,225. 85² → 8 × 9 = 72 →…
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