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      Question

      The average of 15 numbers is 80. The first seven

      numbers and the last five numbers have averages of 90 and 54 respectively. The ninth number exceeds the eighth number by 20%, and the tenth number is reduced by (50/9)% from the ninth number. Determine the value of the ninth number.
      A 84 Correct Answer Incorrect Answer
      B 96 Correct Answer Incorrect Answer
      C 108 Correct Answer Incorrect Answer
      D 120 Correct Answer Incorrect Answer
      E 132 Correct Answer Incorrect Answer

      Solution

      Let eighth number is 10x
      So, ninth number = 1.20 Γ— 10x = 12x
      Tenth number = 850/9 Γ— 1/100 Γ— 12x = 34x/3
      So, 10x + 12x + 34x/3
      = 80 Γ— 15 – 7 Γ— 90 – 5 Γ— 54
      = 1200 – 630 – 270
      = 300
      Or, 100x/3 = 300
      Or, x = 9
      So, ninth number = 12 Γ— 9 = 108

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