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      Question

      If '81x4y' is a five-digit number which is divisible by 72,

      then find the value of (3x + 5y).
      A 20 Correct Answer Incorrect Answer
      B 19 Correct Answer Incorrect Answer
      C 24 Correct Answer Incorrect Answer
      D 30 Correct Answer Incorrect Answer

      Solution

      ATQ,

      Since, 72 = 9 Γ— 8

      So, the given number must be divisible by both 9 and 8.

      Divisibility by 8:

      Last three digits of the number (x4y) must be divisible by 8.

      For x = 3 and y = 2, the number "342" is divisible by 8.

      Divisibility by 9:

      Sum of the digits (8 + 1 + 3 + 4 + 2 = 18) must be divisible by 9, which it is.

      Hence, x = 3, y = 2

      And, 3x + 5y = 3 Γ— 3 + 5 Γ— 2 = 9 + 10 = 19


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