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例如:一項工程,甲獨做10小時完成,乙獨做15小時完成.現在首先由乙先做2小時,再由甲乙合作,還需幾小時就能完成?設還需x小時就能完成,則有方程:x10+x+215=1,解得:x=5.2即5小時12分.
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