The average (arithmetic mean) of y numbers is x. If 30 is added to the set of numbers, then the average will be x - 5. What is the value of y in terms of x ?
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A$$\frac{x}{7}-5$$
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B$$\frac{x}{6}-6$$
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C$$\frac{x}{6}-5$$
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D$$\frac{x}{5}-7$$
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E$$\frac{x}{5}-6$$
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正确答案: D