Bài 1. (3,5 điểm) Hai ca nô làm nhiệm vụ đưa thư giữa hai bến sông A và B dọc theo bờ sông như sau: hàng ngày vào lúc quy định hai ca nô rời bến A và B chạy đến gặp nhau, trao đổi bưu kiện cho nhau rồi quay trở lại. Nếu hai ca nô cùng rời bến một lúc thì ca nô từ A phải đi mất 1,5h mới trở về đến bến, còn ca nô từ B phải đi mất 2,5h. Biết rằng hai ca nô có cùng tốc độ đối với nước v1 không đổi và nước chảy với tốc độ v2 không đổi. Bỏ qua thời gian trao đổi bưu kiện. 1. Tính tốc độ trung bình của mỗi ca nô trên cả quãng đường đi và về. 2. Muốn cho hai ca nô đi mất thời gian như nhau thì ca nô ở B phải xuất phát muộn hơn ca nô ở A một khoảng thời gian bằng bao nhiêu?

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