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簡單,7位數從1000000到9999999 一共有9999999 - 1000000 + 1 = 9000000個. 個位數位和為奇數的、與為偶數的一樣多. 囙此各個數位上數位之和為偶數的7位數有9000000/2 = 4500000個.
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