Overview: 1 questions will be solved this time.Among them
           ☆1 inequalities

[ 1/1Inequality]
    Assignment:Find the solution set of inequality 160 ≤400/(a-20) ≤240 .
    Question type: Inequality
    Solution:
    The inequality can be reduced to 2 inequalities：
        160 ≤400 / ( a - 20 )         (1)
        400 / ( a - 20 ) ≤240         (2)
        From the definition field of divisor
         x - 20 ≠ 0        (3 )

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    From inequality(1)：
         20 ≤ a ≤ 45/2
    From inequality(2)：
         a ≤ 20 或  a ≥ 65/3
    From inequality(3)：
         a < 20 或  a ＞ 20

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    From inequalities (1) and (2)
         20 ≤ a ≤ 20 或  65/3 ≤ a ≤ 45/2    (4)
    From inequalities (3) and (4)
         65/3 ≤ a ≤ 45/2    (5)

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    The final solution set is :
         65/3 ≤ a ≤ 45/2

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