Bài 3: Một cano xuôi dòng trên khúc sông từ A đến B rồi ngay lập tức ngược dòng khúc sông ấy hết tất cả 4 giờ 30 phút. Tính vận tốc riêng của cano biết vận tốc dòng nước là 6km/h.

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