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by soVeryTired
2631 days ago
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In general I find mental arithmetic like this easiest if you start from the most significant digits. So you just need to keep track of a running sum and which multiplications you still need to add. Here's the calculation as I would do it in my head, more or less: 25 * 63 = (20 + 5) * (60 * 3) = (20 * 60) + (5 * 60) + (3 * 20) + (5 * 3) = 1200 + (5 * 60) + (3 * 20) + (5 * 3) = 1500 + (3 * 20) + (5 * 3) = 1560 + (5 * 3) = 1575 |
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