几人？

解答：
168


假設常客人數為x
x = x/2 + x/4 + x/7 + x/12 + 4
x = 42x/84 + 21x/84 + 12x/84 + 7x/84 + 4
x = 82x/84 + 4
x - 82x/84 = 4
84x/84 - 82x/84 = 4
2x/84 = 4
2x = (4 x 84) = 336
x = 336/2 = 168

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