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GMAT prep - Quant question

This topic has 3 expert replies and 0 member replies
NadineKh Newbie | Next Rank: 10 Posts Default Avatar
Joined
28 Jul 2017
Posted:
5 messages

GMAT prep - Quant question

Post Wed Sep 13, 2017 8:17 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    I've had the following question and its answer without explanation. Could someone explain me the solution?

    Thanks!

    For every positive integer n, the function h(n) is defined to be the product of all the even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100)+1, then p is:
    A. Between 2 and 10
    B. Between 10 and 20
    C. Between 20 and 30
    D. Between 30 and 40
    E. Greater than 40

    Answer: E

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    Post Wed Sep 13, 2017 8:19 am
    Quote:
    For every positive even integer n, the function h(n) is defined to be the product of all even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100) + 1, the p is

    A: Between 2 & 10
    B: Between 10 & 20
    C: Between 20 & 30
    D: Between 30 & 40
    E: Greater than 40
    Since the difference between them is 1, h(100) and h(100)+1 are consecutive integers.
    Consecutive integers are COPRIMES: they share no factors other than 1.

    Let's examine why:

    If x is a multiple of 2, the next largest multiple of 2 is x+2.
    If x is a multiple of 3, the next largest multiple of 3 is x+3.

    Using this logic, if we go from x to x+1, we get only to the next largest multiple of 1.
    So 1 is the only factor common both to x and to x+1.
    In other words, x and x+1 are COPRIMES.

    Thus:
    h(100) and h(100)+1 are COPRIMES. They share no factors other than 1.

    h(100) = 2 * 4 * 6 *....* 94 * 96 * 98 * 100
    Factoring out 2 from every value above, we get:
    h(100) = 2⁵⁰(1 * 2 * 3 *... * 47 * 48 * 49 * 50)

    Looking at the set of parentheses on the right, we can see that every prime number between 1 and 50 is a factor of h(100).
    Since h(100) and h(100)+1 are coprimes, NONE of the prime numbers between 1 and 50 can be a factor of h(100)+1.

    Thus, the smallest prime factor of h(100) + 1 must be greater than 50.

    The correct answer is E.

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    Thanked by: NadineKh
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    Post Wed Sep 13, 2017 9:46 am
    Quote:
    For every positive even integer n, the function h(n) is defined to be the product of all even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100) + 1, the p is

    A: Between 2 & 10
    B: Between 10 & 20
    C: Between 20 & 30
    D: Between 30 & 40
    E: Greater than 40
    Important Concept: If integer k is greater than 1, and k is a factor (divisor) of N, then k is not a divisor of N+1
    For example, since 7 is a factor of 350, we know that 7 is not a factor of (350+1)
    Similarly, since 8 is a factor of 312, we know that 8 is not a factor of 313

    Now let’s examine h(100)
    h(100) = (2)(4)(6)(8)….(96)(98)(100)
    = (2x1)(2x2)(2x3)(2x4)....(2x48)(2x49)(2x50)
    Factor out all of the 2's to get: h(100) = [2^50][(1)(2)(3)(4)….(48)(49)(50)]

    Since 2 is in the product of h(100), we know that 2 is a factor of h(100), which means that 2 is not a factor of h(100)+1 (based on the above rule)

    Similarly, since 3 is in the product of h(100), we know that 3 is a factor of h(100), which means that 3 is not a factor of h(100)+1 (based on the above rule)

    Similarly, since 5 is in the product of h(100), we know that 5 is a factor of h(100), which means that 5 is not a factor of h(100)+1 (based on the above rule)

    .
    .
    .
    .
    Similarly, since 47 is in the product of h(100), we know that 47 is a factor of h(100), which means that 47 is not a factor of h(100)+1 (based on the above rule)

    So, we can see that none of the primes from 2 to 47 can be factors of h(100)+1, which means the smallest prime factor of h(100)+1 must be greater than 47.

    Answer = E

    Cheers,
    Brent

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    Post Tue Sep 19, 2017 3:06 pm
    Quote:
    Thanks!

    For every positive integer n, the function h(n) is defined to be the product of all the even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100)+1, then p is:
    A. Between 2 and 10
    B. Between 10 and 20
    C. Between 20 and 30
    D. Between 30 and 40
    E. Greater than 40

    Answer: E
    We are given that h(n) is defined as the product of all the even integers from 2 to n inclusive. For example, h(8) = 2 x 4 x 6 x 8.

    We need to determine the smallest prime factor of h(100) + 1. Before determining the smallest prime factor of h(100) + 1, we must recognize that h(100) and h(100) + 1 are consecutive integers, and consecutive integers will never share the same prime factors.

    Thus, h(100) and h(100) + 1 must have different prime factors. However, rather than determining all the prime factors of h(100), let’s determine the largest prime factor of h(100). Since h(100) is the product of the even integers from 2 to 100 inclusive, let’s find the largest prime number such that 2 times that prime number is less than 100.

    That prime number is 47, since 2 x 47 = 94, which is less than 100. The next prime after 47 is 53, and 2 x 53 = 106, which is greater than 100.

    Therefore, 47 is the largest prime number that is a factor of h(100). In fact, all prime numbers from 2 to 47 are included in the prime factorization of h(100). Since we have mentioned that h(100) + 1 will not have any of the prime factors of h(100), all the prime factors in h(100) + 1, including the smallest one, must be greater than 47. Looking at the answer choices, only choice E can be the correct answer.

    Answer: E

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    Jeffrey Miller Head of GMAT Instruction

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