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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 ## Number N is randomly selected from a set of all primes betwe ##### This topic has 4 expert replies and 0 member replies ## Number N is randomly selected from a set of all primes betwe Number N is randomly selected from a set of all primes between 10 and 40, inclusive. Number K is selected from a set of all multiples of 5 between 10 and 40 inclusive. What is the probability that N+K is odd? (A) 1/2 (B) 2/3 (C) 3/4 (D) 4/7 (E) 5/8 Is there a strategic approach to this question? Can any experts help? ### GMAT/MBA Expert GMAT Instructor Joined 04 Oct 2017 Posted: 551 messages Followed by: 11 members Upvotes: 180 Hello ardz24. First, you have to notice that N is always an odd number. Then, if you want that N+K is odd, then K has to be even (because N is odd). (it doesn't matter how we select N). Now, K has to be selected from {10, 15, 20, 25, 30, 35, 40}. (7 options). So, the favorable cases are k=10, 20, 30, 40. (4 options). The probability that N+K is odd is equal to: $$\frac{Favorable\ options}{Total\ options}=\frac{4}{7}.$$ So, the answer should be D. I hope this can help you. I'm available if you'd like any follow up. Regards. _________________ 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: 13038 messages Followed by: 1251 members Upvotes: 5254 GMAT Score: 770 ardz24 wrote: Number N is randomly selected from a set of all primes between 10 and 40, inclusive. Number K is selected from a set of all multiples of 5 between 10 and 40 inclusive. What is the probability that N+K is odd? (A) 1/2 (B) 2/3 (C) 3/4 (D) 4/7 (E) 5/8 Notice that there are 2 ways that the sum N+K can be ODD 1) N is ODD and K is EVEN 2) N is EVEN and K is ODD However, N cannot be EVEN, since all of the primes between 10 and 40 are ODD (11, 13, 17, 19, . . . . 31, 37) So, case 2 (above) is impossible So, P(N+K is odd) = P(N is odd AND K is even) = P(N is odd) x P(K is even) ------ASIDE------- Possible values of N: (11, 13, 17, 19, . . . . 31, 37) So, P(N is odd) = 1 Possible values of K: (10, 15, 20, 25, 30, 35, 40) So, P(K is even) = 4/7 ----------------------------------------- So, P(N+K is odd) = P(N is odd AND K is even) = P(N is odd) x P(K is even) = 1 x 4/7 = 4/7 = D 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 25 Apr 2015 Posted: 2950 messages Followed by: 19 members Upvotes: 43 ardz24 wrote: Number N is randomly selected from a set of all primes between 10 and 40, inclusive. Number K is selected from a set of all multiples of 5 between 10 and 40 inclusive. What is the probability that N+K is odd? (A) 1/2 (B) 2/3 (C) 3/4 (D) 4/7 (E) 5/8 In order for N + K to be odd, we need an even + odd or odd + even. Since N is a prime between 10 and 40, N must be odd. Thus, we need to determine how many even multiples of 5 there are from 10 to 40 inclusive. We have 10, 20, 30, and 40. We also have (40 - 10)/5 + 1 = 7 total multiples of 5 from 10 to 40 inclusive. Thus, the probability that N is odd and K is even is 1 x 4/7 = 4/7. Answer: D _________________ Scott Woodbury-Stewart Founder and CEO scott@targettestprep.com See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews ### GMAT/MBA Expert Elite Legendary Member Joined 23 Jun 2013 Posted: 10197 messages Followed by: 497 members Upvotes: 2867 GMAT Score: 800 Hi ardz24, We're told that number N is randomly selected from a set of all PRIMES between 10 and 40, inclusive and number K is selected from a set of all multiples of 5 between 10 and 40 inclusive. We're asked for the probability that N+K is ODD. For the SUM to be ODD, one of the numbers must be EVEN and the other must be ODD. Since all of the primes from 10 to 40 are ODD, K MUST be EVEN... sowe're ultimately asked for the probability that K is EVEN... Multiples of 5 from 10 and 40: 10, 15, 20, 25, 30, 35, 40 ---7 numbers Even numbers in this set: 10, 20, 30, 40 -- 4 numbers The probability that K is EVEN (and thus, N+K is ODD) = 4/7 Final Answer: D GMAT assassins aren't born, they're made, Rich _________________ Contact Rich at Rich.C@empowergmat.com • Free Practice Test & Review How would you score if you took the GMAT Available with Beat the GMAT members only code • Magoosh Study with Magoosh GMAT prep Available with Beat the GMAT members only code • 5 Day FREE Trial Study Smarter, Not Harder Available with Beat the GMAT members only code • 1 Hour Free BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • Free Trial & Practice Exam BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • Get 300+ Practice Questions 25 Video lessons and 6 Webinars for FREE Available with Beat the GMAT members only code • FREE GMAT Exam Know how you'd score today for$0

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