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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 × 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 × 3 + 5 × 2 = 9 + 10 = 19


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