Câu 5: (*) Một ôtô đi trên quãng đường S. 1/4 quãng đường đầu ôtô đi với vận tốc 20km/h, trong 2/4 quãng đường kê tiếp ôtô đi với vận tốc 25km/h, trong 1/4 quãng đường cuối cùng ôtô đi với vận tốc 15km/h. Hãy tính vận tốc trung bình của ôtô trên cả quãng đường S. Câu 6: (*) Một người đua xe đạp, trong nửa quãng đường đầu người đó đi với vận tốc 20 km/h. Tính vận tốc của người đó đi trong nửa đoạn đường còn lại. Biết rằng vận tốc trung bình trong cả đoạn đường là 23 km/h.

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