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What is the smallest integer that. . . . .

This topic has 2 expert replies and 0 member replies

What is the smallest integer that. . . . .

Post Wed Sep 27, 2017 6:12 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    What is the smallest integer that can be divided by the product of a prime number and 7 while yielding a prime number?

    (A) 7
    (B) 14
    (C) 24
    (D) 28
    (E) 35

    The OA is D.

    I did not understand the question. What do I need to calculate here? I am confuse.

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    Post Wed Sep 27, 2017 7:56 pm
    Hi Vincen,

    We're asked to find the smallest integer that can be DIVIDED by the PRODUCT of a PRIME number and 7 AND will yield a PRIME number as the result. We can answer this question by TESTing THE ANSWERS.

    Since we're asked for the SMALLEST number that will fit the above information, we should start with Answer A: 7....

    The smallest product that can occur between 7 and a prime is (7)(2) = 14. 7 is NOT divisibly by 14, so we can eliminate Answer A.

    Answer B: 14...

    While 14 is divisible by 14, the end result is 1 - but that is NOT a prime number. Eliminate Answer B.

    Answer C: 24

    24/14 is NOT a prime. The next two products to consider would be (7)(3) = 21 and (7)(5) = 35. However, 24/21 and 24/35 are NOT primes. Eliminate Answer C.

    Answer D: 28
    28/14 = 2 - and that IS a prime. This matches everything we were told, so this must be the answer.

    Final Answer: D

    GMAT assassins aren't born, they're made,
    Rich

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    GMAT/MBA Expert

    Post Thu Sep 28, 2017 4:30 pm
    They're saying that

    the answer / (7 * some prime) = another prime

    And they want the answer to be the smallest number that solves the equation above.

    If we call the answer a, some prime p, and the other prime q, we've got:

    a / 7p = q

    a = 7pq

    To make this as small as possible, make p and q the smallest primes possible: p = 2, q = 2.

    From that, a = 7 * 2 * 2 = 28

    Thanked by: Vincen
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