In a certain class, x students are 10 years old, and the remaining y students are 11 years old. What is the average (arithmetic mean) age of all students in the class?
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A$${x+y}\over{21}$$
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B$${10x+11y}\over{21}$$
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C$${110xy}\over{x+y}$$
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D$${110xy}\over{xy}$$
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E$${10x+11y}\over{x+y}$$
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正确答案: E