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Register now and save up to $200 Available with Beat the GMAT members only code • Free Practice Test & Review How would you score if you took the GMAT Available with Beat the GMAT members only code • Free Veritas GMAT Class Experience Lesson 1 Live Free Available with Beat the GMAT members only code • Magoosh Study with Magoosh GMAT prep Available with Beat the GMAT members only code • Reach higher with Artificial Intelligence. Guaranteed Now free for 30 days 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 • 5-Day Free Trial 5-day free, full-access trial TTP Quant 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 ## OG-17 This topic has 4 expert replies and 0 member replies Joy Shaha Senior | Next Rank: 100 Posts Joined 05 May 2016 Posted: 59 messages Upvotes: 3 #### OG-17 Sat Nov 19, 2016 1:14 pm The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur? A) 1/5 B) 2/5 C) 3/5 D) 4/5 E) 12/25 ### GMAT/MBA Expert Jay@ManhattanReview GMAT Instructor Joined 22 Aug 2016 Posted: 1029 messages Followed by: 22 members Upvotes: 470 Sun Dec 11, 2016 11:33 pm Joy Shaha wrote: The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur? A) 1/5 B) 2/5 C) 3/5 D) 4/5 E) 12/25 Hi Joy, Given that: Probability of event M NOT to occur = P(M') = 0.8 => Probability of event M to occur = P(M) = 1 - 0.8 = 0.2 Probability of event R NOT to occur = P(R') = 0.6 => Probability of event R to occur = P(R) = 1 - 0.6 = 0.4 We are given that either of the two events can occur, thus, (Probability of M to occur * Probability of R NOT to occur) + (Probability of M NOT to occur * Probability of R to occur) = P(M)*P(R') + P(M')*P(R) = 0.2*0.6 + 0.8*0.4 = 0.12 + 0.48 = 0.60 = 3/5. Hope this helps! -Jay _________________ Manhattan Review GMAT Prep Locations: New York | Mumbai | Ho Chi Minh City | Budapest | and many more... Schedule your free consultation with an experienced GMAT Prep Advisor! Click here. ### GMAT/MBA Expert Scott@TargetTestPrep GMAT Instructor Joined 25 Apr 2015 Posted: 891 messages Followed by: 6 members Upvotes: 43 Sun Nov 27, 2016 5:51 pm Joy Shaha wrote: The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur? A) 1/5 B) 2/5 C) 3/5 D) 4/5 E) 12/25 We are given that the probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6, and that events M and R cannot both occur. We need to determine the probability that either event M or event R will occur. The probability that event M will occur is 1 - 0.8 = 0.2 = 1/5 The probability that event R will occur is 1 - 0.6 = 0.4 = 2/5 Recall that the formula for the probability of event A or event B occurring is P(A or B) = P(A) + P(B) - P(A and B); therefore: P(M or R) = P(M) + P(R) - P(M and R). Since we are told that events M and R cannot both occur, that means P(M and R) = 0 and thus the probability that either event M or event R will occur is: P(M or R) = 1/5 + 2/5 - 0 = 3/5. Answer:C _________________ Scott Woodbury-Stewart Founder and CEO ### GMAT/MBA Expert DavidG@VeritasPrep Legendary Member Joined 14 Jan 2015 Posted: 2667 messages Followed by: 120 members Upvotes: 1153 GMAT Score: 770 Sat Nov 19, 2016 2:48 pm Joy Shaha wrote: The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur? A) 1/5 B) 2/5 C) 3/5 D) 4/5 E) 12/25 If the probability that M will not occur is .8, the probability that it will occur is .2. If the probability that R will not occur is .6, the probability that it will occur is .4. So P(M) = .2 and P(R) = .4. P(M or R) = P(M) + P(R) - P(M and R). Because we're told that P(M and R) = 0, we have P(M or R) = .2 + .4 = .6 = 3/5. The answer is C _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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Matt@VeritasPrep GMAT Instructor
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Fri Nov 25, 2016 3:22 pm
It might be easier to see this using less abstract terms.

Suppose I told you that the Mariners have a 20% chance of winning the World Series, and that the Rangers have a 40% chance of winning the World Series. Since at most one of these teams could win, the probability that the Mariners OR the Rangers win = 20% + 40% = 60%.

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