Someone walked to school. The average speed of her first 30 miles was x, and the average speed of her later 60 miles was y. What's the average speed of this person on this 90 miles road?
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A$$\frac{x+y}{2}$$
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Bx+$$\frac{y}{2}$$
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C$$\frac{x}{2}$$+y
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D$$\frac{2x+y}{3xy}$$
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E$$\frac{3xy}{2x+y}$$
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正确答案: E