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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## If the sum of two prime numbers x and y is odd, tagged by: VJesus12 ##### This topic has 6 expert replies and 0 member replies ### Top Member ## If the sum of two prime numbers x and y is odd, If the sum of two prime numbers x and y is odd, then the product of x and y must be divisible by (A) 2 (B) 3 (C) 4 (D) 5 (E) 8 The OA is A. I got confused here. Can be the sum of two odd numbers and odd number? the prime numbers are all odd. I really need some help here please. ### GMAT/MBA Expert GMAT Instructor Joined 08 Dec 2008 Posted: 13036 messages Followed by: 1251 members Upvotes: 5254 GMAT Score: 770 VJesus12 wrote: If the sum of two prime numbers x and y is odd, then the product of x and y must be divisible by (A) 2 (B) 3 (C) 4 (D) 5 (E) 8 Some important rules: 1. ODD + ODD = EVEN 2. ODD + EVEN = ODD 3. EVEN + EVEN = EVEN 4. (ODD)(ODD) = ODD 5. (ODD)(EVEN) = EVEN 6. (EVEN)(EVEN) = EVEN If x + y = ODD, then one of the values must be ODD and the other value must be EVEN Since 2 is the ONLY EVEN prime number, then we can be certain that either x or y equals 2. So, xy = (2)(some ODD prime number) In other words, xy = (EVEN )(ODD) = EVEN (from rule 5) So, xy is EVEN, which means xy is divisible by 2 Answer: A Cheers, Brent _________________ Brent Hanneson â€“ Creator of GMATPrepNow.com Use my video course along with Sign up for free Question of the Day emails And check out all of these free resources GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMATâ€™s FREE 60-Day Study Guide and reach your target score in 2 months! ### GMAT/MBA Expert GMAT Instructor Joined 04 Oct 2017 Posted: 551 messages Followed by: 11 members Upvotes: 180 Hello Vjesus12. First of all, not all the prime numbers are odd numbers. The number 2 is the only prime number which is an even number. Now, if we add two odd numbers the result will be an even number. This is not our situation. We have the result of the sum is an odd number. The only way to get this is adding an even number plus and odd number. So, one of the prime numbers is even. It implies one of the prime number is 2. Let's say x=2. Now, we have that x*y=2*y. So the product is an even number. In conclusion the product of x by y is divisible by 2. The correct answer is A. I hope it helps you. I'm available if you'd like any follow up. _________________ GMAT Prep From The Economist We offer 70+ point score improvement money back guarantee. Our average student improves 98 points. Free 7-Day Test Prep with Economist GMAT Tutor - Receive free access to the top-rated GMAT prep course including a 1-on-1 strategy session, 2 full-length tests, and 5 ask-a-tutor messages. Get started now. ### GMAT/MBA Expert GMAT Instructor Joined 08 Dec 2008 Posted: 13036 messages Followed by: 1251 members Upvotes: 5254 GMAT Score: 770 VJesus12 wrote: If the sum of two prime numbers x and y is odd, then the product of x and y must be divisible by (A) 2 (B) 3 (C) 4 (D) 5 (E) 8 Alternatively, we can use the process of elimination to solve the question. The question asks "the product of x and y must be divisible by..." So, if we can show that the product xy is NOT divisible by a certain answer choice, then we can ELIMINATE that answer choice. GIVEN: the sum of two prime numbers x and y is odd So, for example, x = 2 and y = 7 satisfies the given information. xy = (2)(7) = 14, so it's possible that xy = 14 Now check the answer choices.... (A) 2. 14 IS divisible by 2. So, KEEP A for now. (B) 3. 14 is NOT divisible by 3. ELIMINATE B (C) 4. 14 is NOT divisible by 4. ELIMINATE C (D) 5. 14 is NOT divisible by 5. ELIMINATE D (E) 8. 14 is NOT divisible by 8. ELIMINATE E By the process of elimination, the correct answer is A Cheers, Brent _________________ Brent Hanneson â€“ Creator of GMATPrepNow.com Use my video course along with Sign up for free Question of the Day emails And check out all of these free resources GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMATâ€™s FREE 60-Day Study Guide and reach your target score in 2 months! ### GMAT/MBA Expert Elite Legendary Member Joined 23 Jun 2013 Posted: 10197 messages Followed by: 497 members Upvotes: 2867 GMAT Score: 800 Hi VJesus12, This question is all about Number Property rules, so you can either TEST VALUES or use your knowledge of those NP rules to answer the question. As an aside, the fact that X and Y are PRIME is actually irrelevant to this question. If the two variables are positive integers, and their sum is ODD, then one of the variables MUST be EVEN and the other MUST be ODD. The product of an even and an odd is ALWAYS EVEN. By extension, even numbers are ALWAYS divisible by 2. Final Answer: A GMAT assassins aren't born, they're made, Rich _________________ Contact Rich at Rich.C@empowergmat.com ### GMAT/MBA Expert GMAT Instructor Joined 04 Oct 2017 Posted: 551 messages Followed by: 11 members Upvotes: 180 Hi VJesus12, Let's take a look at your question. The question states that the sum of two prime numbers x and y is odd. Sum of two numbers is odd only when one number is even and the other is odd. SUM (ODD) = EVEN NUMBER + ODD NUMBER Now, we will see if the product of an even number and an odd number is even or odd? (EVEN NUMBER)(ODD NUMBER) = ? For example, even number = 8 and odd number = 3 then product = 8(3) = 24, which is an even number. Therefore, (EVEN NUMBER)(ODD NUMBER) = EVEN NUMBER And an even number is always divisible by 2. Therefore, Option A is correct. Hope this helps. I am available, if you'd like any follow up. _________________ GMAT Prep From The Economist We offer 70+ point score improvement money back guarantee. Our average student improves 98 points. Free 7-Day Test Prep with Economist GMAT Tutor - Receive free access to the top-rated GMAT prep course including a 1-on-1 strategy session, 2 full-length tests, and 5 ask-a-tutor messages. Get started now. ### GMAT/MBA Expert GMAT Instructor Joined 09 Apr 2015 Posted: 1465 messages Followed by: 19 members Upvotes: 39 VJesus12 wrote: If the sum of two prime numbers x and y is odd, then the product of x and y must be divisible by (A) 2 (B) 3 (C) 4 (D) 5 (E) 8 All prime numbers are odd, except for 2. Recall that odd + odd = even and that even + odd = odd. If the sum of two prime numbers is odd, then we see that one of the numbers must be 2 and the other number must be odd. Thus, the product xy is a multiple of 2 and must be divisible by 2. 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