Bạn An đi xe đạp từ nhà tới trường với vận tốc 12km/h.Khi tới trường bạn học thể dục trong 1 giờ rồi quay trở về nhà trên con đường cũ với vận tốc 15km/h.Tính quãng đường từ nhà tới trường của bạn An,biết thời gian cả đi và về hết 2 giờ 30 phút (kể cả thời gian học thể dục)

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