Bài 3: Hai người cùng làm chung một công việc. Nếu làm riêng, người thứ nhất phải mất 4 giờ, người thứ hai phải mất 7 giờ mới xong công việc. Hỏi nếu làm chung thì mỗi giờ cả hai người làm được mấy phần công việc? Bài 4: Hai vòi cùng chảy vào 1 bé. Nếu vòi thứ nhất chảy thì phải mất 4 giờ 25 phút mới đầy bể. Nếu vòi thứ hai chảy thì phải mất 8 giờ 12 phút mới đầy bể. Hỏi trong 1 giờ, hai vòi chảy được bao nhiêu phần bể?

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