cùng 1 lúc tại 2 địa điểm A và B trên một đường thẳng có hai xe khởi hành chuyển động cùng chiều.Xe A có vận tốc 40km/h,đuổi theo xe B đang chạy với vận tốc 20km/h.Tìm vận tốc của xe A đối với xe B và thời gian đi để xe A đuổi kịp xe B .biết khoảng cách AB bằng 30km

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