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x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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23 Jul 2013, 07:44
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x is the product of all even numbers from 2 to 50, inclusive. The smallest prime factor of x+1 must be (A) Between 1 and 10 (B) Between 11 and 15 (C) Between 15 and 20 (D) Between 20 and 25 (E) Greater than 25 My Question: Please provide an explanation on how to arrive at the answer.
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Re: x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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23 Jul 2013, 07:57
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hb wrote: x is the product of all even numbers from 2 to 50, inclusive. The smallest prime factor of x+1 must be (A) Between 1 and 10 (B) Between 11 and 15 (C) Between 15 and 20 (D) Between 20 and 25 (E) Greater than 25 My Question: Please provide an explanation on how to arrive at the answer. Disclaimer: I have used the Search Box Before Posting. I used the first sentence of the question or a string of words exactly as they show up in the question below for my search. I did not receive an exact match for my question. Source: Veritas Prep; Book 02 Chapter: Homework Topic: Arithmetic Question: 105 Question: Page 251 Solution: PDF Page 20 of 32 \(x=2*4*6*...*50=(2*1)*(2*2)*(2*3)*...*(2*25)=2^{25}(1*2*3*...*25)=2^{25}*25!\). This number is obviously divisible by each prime less than 25. Now, x and x+1 are consecutive integers. Two consecutive integers are coprime, which means that they don't share ANY common factor but 1. For example 20 and 21 are consecutive integers, thus only common factor they share is 1. Since x has all prime numbers from 1 to 25 as its factors, according to above x+1 won't have ANY prime factors from 1 to 25. Hence the smallest prime factor of x+1 will be greater than 25. Answer: E. Similar questions to practice: foreverypositiveevenintegernthefunctionhnis126691.htmlforeverypositiveevenintegernthefunctionhn149722.htmlifnisapositiveintegergreaterthan1thenpnreprese144553.htmlHope it helps.
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Re: x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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23 Jul 2013, 08:00
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hb wrote: Disclaimer: I have used the Search Box Before Posting. I used the first sentence of the question or a string of words exactly as they show up in the question below for my search. I did not receive an exact match for my question. Source: Veritas Prep; Book 02 Chapter: Homework Topic: Arithmetic Question: 105 Question: Page 251 Solution: PDF Page 20 of 32 Questioin 105. x is the product of all even numbers from 2 to 50, inclusive. The smallest prime factor of x+1 must be (A): Between 1 and 10 (B): Between 11 and 15 (C): Between 15 and 20 (D): Between 20 and 25 (E): Greater than 25 My Question: Please provide an explanation on how to arrive at the answer. SOME THEORY FIRST: HAVE A LOOK OF THIS PATTERN. Factorial 2 +1=3 smallest factor is 3==>this is greater than 2 factorial 3 + 1=7 smallest factor is 7==>this is greater than 3 factorial 4 + 1 = 25 smallest factor is 5==>which is greater than 4 factorial 5 + 1= 121 smallest factor is 11==>which is greater than 5 (note: i am excluding 1 as a smallest factor) now by seeing this pattern you can figure out that.. (factorial x + 1)==>smallest factor will always be greater than x now coming to our problem all even no.s between 2 to 50 2*4*6*8......*48*50 now take 2 from each number common. 2^25(1*2*3*4*5*......25) or 2^25*factorial 25 now x+1= 2^25*factorial 25 + 1===>clearly smallest factor will be greater than 25(as proved above) hence E hope it helPS
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Re: x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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29 Nov 2015, 10:59
Hi All, The above prompt is essentially just a 'lift' of the following GMAC question (but the concept is exactly the same): For every positive even 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: 1. between 2 and 10 2. between 10 and 20 3. between 20 and 30 4. between 30 and 40 5. greater than 40 The main idea behind this prompt is: "The ONLY number that will divide into X and (X+1) is 1." In other words, NONE of the factors of X will be factors of X+1, EXCEPT for the number 1. Here are some examples: X = 2 X+1 = 3 Factors of 2: 1 and 2 Factors of 3: 1 and 3 ONLY the number 1 is a factor of both. X = 9 X+1 = 10 Factors of 9: 1, 3 and 9 Factors of 10: 1, 2, 5 and 10 ONLY the number 1 is a factor of both. Etc. Since the H(100) is (100)(98)(96)....(4)(2)....we can deduce.... 1) This product will have LOTS of different factors 2) NONE of those factors will divide into H(100) + 1. H(100) contains all of the primes from 2 through 47, inclusive (the 47 can be "found" in the "94"), so NONE of those will be in H(100) + 1. We don't even have to calculate which prime factor is smallest in H(100) + 1; we know that it MUST be a prime greater than 47....and there's only one answer that fits. Final Answer: GMAT assassins aren't born, they're made, Rich
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x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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12 Oct 2017, 13:27
Bunuel wrote: hb wrote: x is the product of all even numbers from 2 to 50, inclusive. The smallest prime factor of x+1 must be (A) Between 1 and 10 (B) Between 11 and 15 (C) Between 15 and 20 (D) Between 20 and 25 (E) Greater than 25 My Question: Please provide an explanation on how to arrive at the answer. Disclaimer: I have used the Search Box Before Posting. I used the first sentence of the question or a string of words exactly as they show up in the question below for my search. I did not receive an exact match for my question. Source: Veritas Prep; Book 02 Chapter: Homework Topic: Arithmetic Question: 105 Question: Page 251 Solution: PDF Page 20 of 32 \(x=2*4*6*...*50=(2*1)*(2*2)*(2*3)*...*(2*25)=2^{25}(1*2*3*...*25)=2^{25}*25!\). This number is obviously divisible by each prime less than 25. Now, x and x+1 are consecutive integers. Two consecutive integers are coprime, which means that they don't share ANY common factor but 1. For example 20 and 21 are consecutive integers, thus only common factor they share is 1. Since x has all prime numbers from 1 to 25 as its factors, according to above x+1 won't have ANY prime factors from 1 to 25. Hence the smallest prime factor of x+1 will be greater than 25. Answer: E. Similar questions to practice: http://gmatclub.com/forum/foreverypos ... 26691.htmlhttp://gmatclub.com/forum/foreverypos ... 49722.htmlhttp://gmatclub.com/forum/ifnisapos ... 44553.htmlHope it helps. hi Bunuel since x is the product of all even integers from 2 to 50 inclusive, x = (2 * 4 * 6 * 8 * 10 * 12 * 14.....* 50) which can be rewriten as 2( 1 * 2 * 3 * 4 * 5 * 6 * 7 *........* 25) so, x is equal to 2 * 25! please say to me why this is not okay ... thanks in advance, man



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Re: x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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12 Oct 2017, 13:51
gmatcracker2017 wrote: Bunuel wrote: hb wrote: x is the product of all even numbers from 2 to 50, inclusive. The smallest prime factor of x+1 must be (A) Between 1 and 10 (B) Between 11 and 15 (C) Between 15 and 20 (D) Between 20 and 25 (E) Greater than 25 My Question: Please provide an explanation on how to arrive at the answer. Disclaimer: I have used the Search Box Before Posting. I used the first sentence of the question or a string of words exactly as they show up in the question below for my search. I did not receive an exact match for my question. Source: Veritas Prep; Book 02 Chapter: Homework Topic: Arithmetic Question: 105 Question: Page 251 Solution: PDF Page 20 of 32 \(x=2*4*6*...*50=(2*1)*(2*2)*(2*3)*...*(2*25)=2^{25}(1*2*3*...*25)=2^{25}*25!\). This number is obviously divisible by each prime less than 25. Now, x and x+1 are consecutive integers. Two consecutive integers are coprime, which means that they don't share ANY common factor but 1. For example 20 and 21 are consecutive integers, thus only common factor they share is 1. Since x has all prime numbers from 1 to 25 as its factors, according to above x+1 won't have ANY prime factors from 1 to 25. Hence the smallest prime factor of x+1 will be greater than 25. Answer: E. Similar questions to practice: http://gmatclub.com/forum/foreverypos ... 26691.htmlhttp://gmatclub.com/forum/foreverypos ... 49722.htmlhttp://gmatclub.com/forum/ifnisapos ... 44553.htmlHope it helps. hi Bunuel since x is the product of all even integers from 2 to 50 inclusive, x = (2 * 4 * 6 * 8 * 10 * 12 * 14.....* 50) which can be rewriten as 2( 1 * 2 * 3 * 4 * 5 * 6 * 7 *........* 25) so, x is equal to 2 * 25! please say to me why this is not okay ... thanks in advance, man gmatcracker2017: I believe you are mixing the rule up with a question where we are talking about the sum of all even integers between 2 and 50. Then it would be 2+3+6...+50 which can factor out just one 2, but since it's the product, you have to factor out all 25 "2"s. I believe that is where your thought processes is getting confused, but mine gets confused often, so Bunuel may be best to confirm.



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Re: x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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12 Oct 2017, 14:13
gmatcracker2017 wrote: Bunuel wrote: hb wrote: x is the product of all even numbers from 2 to 50, inclusive. The smallest prime factor of x+1 must be (A) Between 1 and 10 (B) Between 11 and 15 (C) Between 15 and 20 (D) Between 20 and 25 (E) Greater than 25 My Question: Please provide an explanation on how to arrive at the answer. Disclaimer: I have used the Search Box Before Posting. I used the first sentence of the question or a string of words exactly as they show up in the question below for my search. I did not receive an exact match for my question. Source: Veritas Prep; Book 02 Chapter: Homework Topic: Arithmetic Question: 105 Question: Page 251 Solution: PDF Page 20 of 32 \(x=2*4*6*...*50=(2*1)*(2*2)*(2*3)*...*(2*25)=2^{25}(1*2*3*...*25)=2^{25}*25!\). This number is obviously divisible by each prime less than 25. Now, x and x+1 are consecutive integers. Two consecutive integers are coprime, which means that they don't share ANY common factor but 1. For example 20 and 21 are consecutive integers, thus only common factor they share is 1. Since x has all prime numbers from 1 to 25 as its factors, according to above x+1 won't have ANY prime factors from 1 to 25. Hence the smallest prime factor of x+1 will be greater than 25. Answer: E. Similar questions to practice: http://gmatclub.com/forum/foreverypos ... 26691.htmlhttp://gmatclub.com/forum/foreverypos ... 49722.htmlhttp://gmatclub.com/forum/ifnisapos ... 44553.htmlHope it helps. hi Bunuel since x is the product of all even integers from 2 to 50 inclusive, x = (2 * 4 * 6 * 8 * 10 * 12 * 14.....* 50) which can be rewriten as 2( 1 * 2 * 3 * 4 * 5 * 6 * 7 *........* 25) so, x is equal to 2 * 25! please say to me why this is not okay ... thanks in advance, man hi zflodeen oh! I got it ... I must be sleeping as I posted this ... thanks



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Re: x is the product of all even numbers from 2 to 50, inclusive [#permalink]
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12 Oct 2017, 14:20
No worries gmatcracker2017. Happens all the time to me!




Re: x is the product of all even numbers from 2 to 50, inclusive
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