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      Question

      If 'a' is the mean proportion of 12.8 and 0.8 and 'b' is

      the third proportional to 1.2 and 4.8, then find (a + b) :(b - a) .
      A 5:4 Correct Answer Incorrect Answer
      B 3:5 Correct Answer Incorrect Answer
      C 7:5 Correct Answer Incorrect Answer
      D 6:5 Correct Answer Incorrect Answer

      Solution

      Mean proportion of two numbers 'p' and 'q' is √(p X q) .

      So, 'a' = √(12.8 X 0.8)

      Or, 'a' = √10.24 = 3.2

      Third proportional to two numbers 'p' and 'q' is (q2/p) .

      So, 'b' = (4.82/1.2)

      Or, 'b' = (23.04/1.2) = 19.2

      Required ratio = (3.2 + 19.2) :(19.2 - 3.2) = 22.4:16 = 7:5

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