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COMEDK202510 May 2025Morning ShiftMathematicsSequences and SeriesActual

The digits of a three-digit number taken in an order are in geometric progression. If one is added to the middle digit, they form an arithmetic progression. If 594 is subtracted from the number, then a new number with the same digits in reverse order is formed. The original number is divisible by

Options

  1. A4
  2. B19
  3. C11
  4. D421

Correct answer

D. 421

Step-by-step solution

Let the digits be a, ar, ar^2 in GP. The number is 100a + 10ar + ar^2 . AP condition ( a, b+1, c in AP): 2(ar+1) = a + ar^2 2ar + 2 = a(1+r^2) ... (1) Reverse condition: (100a + 10ar + ar^2) - 594 = 100ar^2 + 10ar + a 99a - 99ar^2 = 594 a(1-r^2) = 6 ... (2) From (1): a - ar^2 = 2 - 2ar . Substituting into (2): 2 - 2ar = 6 ar = -2 Since digits must be positive and a > c (as 594 is subtracted), try r = 1 2 , a = 8 : b = 8 1 2 = 4 , c = 8 1 4 = 2 Verification: 8, 4, 2 are in GP. 8, 5, 2 are in AP. 842 - 594 = 248 (rev

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