Author 
Message 
TAGS:

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 46319

Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
06 Mar 2017, 01:47
Question Stats:
23% (00:58) correct 77% (01:28) wrong based on 276 sessions
HideShow timer Statistics



Senior Manager
Joined: 06 Dec 2016
Posts: 251

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
06 Mar 2017, 11:31
I'm not sure if my approach is right.
6/36 x 6/36 = 1/6 x 1 /6 = 1/36. I think the answer is B. I am not sure though.



Intern
Joined: 05 Sep 2016
Posts: 21
Location: Israel
Concentration: Economics, Technology
WE: Engineering (Telecommunications)

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
06 Mar 2017, 11:47
The answer should be D. 11/216. There are 2 cases: 1. Both dices show the same number, so it's 6 out of 36. Then for the other player the probability to roll the same number is 1/6 x 1/6. 2. There are 30 out of 36 options for the dices to be different numbers. The probability for the other player to roll the same is 2/6 x 1/6. Probability = 1/6 x 1/6 x 1/6 + 5/6 x 2/6 x 1/6 = 11/216 Sent from my Redmi 4 using GMAT Club Forum mobile app



Senior SC Moderator
Joined: 14 Nov 2016
Posts: 1314
Location: Malaysia

Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
08 Mar 2017, 01:38
kolodits wrote: The answer should be D. 11/216. There are 2 cases: 1. Both dices show the same number, so it's 6 out of 36. Then for the other player the probability to roll the same number is 1/6 x 1/6. 2. There are 30 out of 36 options for the dices to be different numbers. The probability for the other player to roll the same is 2/6 x 1/6. \(Probability = \frac{1}{6} * \frac{1}{6} * \frac{1}{6} + \frac{5}{6} * \frac{2}{6} * \frac{1}{6} = \frac{11}{216}\) Sent from my Redmi 4 using GMAT Club Forum mobile appDear kolodits, How do you able to get the value for \(\frac{2}{6}\)?
_________________
"Be challenged at EVERY MOMENT."
“Strength doesn’t come from what you can do. It comes from overcoming the things you once thought you couldn’t.”
"Each stage of the journey is crucial to attaining new heights of knowledge."
Rules for posting in verbal forum  Please DO NOT post short answer in your post!



Intern
Joined: 05 Sep 2016
Posts: 21
Location: Israel
Concentration: Economics, Technology
WE: Engineering (Telecommunications)

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
08 Mar 2017, 06:42
ziyuen wrote: kolodits wrote: The answer should be D. 11/216. There are 2 cases: 1. Both dices show the same number, so it's 6 out of 36. Then for the other player the probability to roll the same number is 1/6 x 1/6. 2. There are 30 out of 36 options for the dices to be different numbers. The probability for the other player to roll the same is 2/6 x 1/6. \(Probability = \frac{1}{6} * \frac{1}{6} * \frac{1}{6} + \frac{5}{6} * \frac{2}{6} * \frac{1}{6} = \frac{11}{216}\) Sent from my Redmi 4 using GMAT Club Forum mobile appDear kolodits, How do you able to get the value for \(\frac{2}{6}\)? The second player roll the first dice. It should be equal to one of the dices the first player rolled. It doesn't matter which one, so there are 2 options out of 6. For the second dice there is only 1 option left, which makes it: 2/6 * 1/6. Sent from my Redmi 4 using GMAT Club Forum mobile app



Intern
Joined: 25 Dec 2016
Posts: 17
Location: United States (GA)
Concentration: Healthcare, Entrepreneurship
GPA: 3.64
WE: Medicine and Health (Health Care)

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
08 Mar 2017, 07:50
At first I misread and thought it was asking for the same sum of the pairs of dice which would yield a different answer.
There are two cases. The first is when the first person rolls two different numbers. This happens 5/6 of the time. The second is when the first person rolls the same number on both die. This happens 1/6 of the time.
In the first case, the second person has a 2/6 chance to roll one of the two numbers that the first person rolled with the first die. If the first die is one of the two numbers, then the second person only has a 1/6 chance for the second die to match up. This means that in the first case, the second person has a 2/6 * 1/6 = 1/18 chance to have the same roll. This applies 5/6 of the time.
In the second case, both dice must match, so the second person has a 1/6 chance to roll the same number with each of the two die. This means the total probability is 1/6 * 1/6 = 1/36. This applies 1/6 of the time.
This means that the total probability is 5/6*1/18 + 1/6*1/36 = 5/108 + 1/216 = 10/216 + 1/216 = 11/216. Answer is D



Target Test Prep Representative
Status: Head GMAT Instructor
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 2570

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
09 Mar 2017, 17:22
Bunuel wrote: Pam and Robin each roll a pair of fair, sixsided dice. What is the probability that Pam and Robin will both roll the same set of two numbers?
A. 1/216 B. 1/36 C. 5/108 D. 11/216 E. 1/18 We need to determine the probability that when Pam and Robin each rolls a pair of fair, sixsided dice, they both roll the same set of numbers. There are two scenarios: when Pam and Robin both roll the same two numbers and when they roll two distinct numbers. Scenario 1: When the two numbers on the dice are the same Let’s say they both roll 1s. That is, Pam rolls (1, 1) and Robin rolls (1, 1). The probability of this happening is 1/6 x 1/6 x 1/6 x 1/6 = 1/(6^4) Since the probability is the same for all 6 pairs of numbers, the probability of their rolling the same numbers is 6 x 1/(6^4) = 1/(6^3) = 1/216. Scenario 2: When the two numbers on the dice are distinct There are 6 x 5 = 30 ways to roll two distinct numbers when rolling two dice. Let’s say Pam rolls (1, 2) and Robin also rolls (1, 2). The probability of this happening is: 1/6 x 1/6 x 1/6 x 1/6 = 1/(6^4) However, if Pam rolls (1, 2) and Robine rolls (2,1), those are still considered the same set of numbers, and the probability of that occurring is also 1/(6^4). Therefore, for each pair of distinct numbers rolled, the probability is 2 x 1/(6^4) = 2/(6^4). Since there are 30 such pairs, the overall probability is 30 x 2/(6^4) = 60/(^4) = 10/(6^3) = 10/216. Finally, since the events in option 1 and those in option 2 are mutually exclusive, we use the addition rule of probability. That is, the probability that Pam and Robin will both roll the same set of two numbers is: 1/216 + 10/216 = 11/216 Answer: D
_________________
Jeffery Miller
Head of GMAT Instruction
GMAT Quant SelfStudy Course
500+ lessons 3000+ practice problems 800+ HD solutions



Intern
Joined: 08 Jul 2016
Posts: 13

Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
26 Mar 2017, 20:58
JeffTargetTestPrep wrote: Bunuel wrote: Pam and Robin each roll a pair of fair, sixsided dice. What is the probability that Pam and Robin will both roll the same set of two numbers?
A. 1/216 B. 1/36 C. 5/108 D. 11/216 E. 1/18 We need to determine the probability that when Pam and Robin each rolls a pair of fair, sixsided dice, they both roll the same set of numbers. There are two scenarios: when Pam and Robin both roll the same two numbers and when they roll two distinct numbers. Scenario 1: When the two numbers on the dice are the same Let’s say they both roll 1s. That is, Pam rolls (1, 1) and Robin rolls (1, 1). The probability of this happening is 1/6 x 1/6 x 1/6 x 1/6 = 1/(6^4) Since the probability is the same for all 6 pairs of numbers, the probability of their rolling the same numbers is 6 x 1/(6^4) = 1/(6^3) = 1/216. Scenario 2: When the two numbers on the dice are distinct There are 6 x 5 = 30 ways to roll two distinct numbers when rolling two dice. Let’s say Pam rolls (1, 2) and Robin also rolls (1, 2). The probability of this happening is: 1/6 x 1/6 x 1/6 x 1/6 = 1/(6^4) However, if Pam rolls (1, 2) and Robine rolls (2,1), those are still considered the same set of numbers, and the probability of that occurring is also 1/(6^4). Therefore, for each pair of distinct numbers rolled, the probability is 2 x 1/(6^4) = 2/(6^4). Since there are 30 such pairs, the overall probability is 30 x 2/(6^4) = 60/(^4) = 10/(6^3) = 10/216. Finally, since the events in option 1 and those in option 2 are mutually exclusive, we use the addition rule of probability. That is, the probability that Pam and Robin will both roll the same set of two numbers is: 1/216 + 10/216 = 11/216 Answer: D If we are already considering (1,2) and (2,1) to be the same set while calculating probability then why do we have total number of pairs = 30? Shouldn't it be 15?



Manager
Joined: 01 Feb 2015
Posts: 73

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
15 Aug 2017, 01:36
jpeeples85 wrote: At first I misread and thought it was asking for the same sum of the pairs of dice which would yield a different answer.
There are two cases. The first is when the first person rolls two different numbers. This happens 5/6 of the time. The second is when the first person rolls the same number on both die. This happens 1/6 of the time.
In the first case, the second person has a 2/6 chance to roll one of the two numbers that the first person rolled with the first die. If the first die is one of the two numbers, then the second person only has a 1/6 chance for the second die to match up. This means that in the first case, the second person has a 2/6 * 1/6 = 1/18 chance to have the same roll. This applies 5/6 of the time.
In the second case, both dice must match, so the second person has a 1/6 chance to roll the same number with each of the two die. This means the total probability is 1/6 * 1/6 = 1/36. This applies 1/6 of the time.
This means that the total probability is 5/6*1/18 + 1/6*1/36 = 5/108 + 1/216 = 10/216 + 1/216 = 11/216. Answer is D 1/6 of the time? 5/6 of the time? How are you saying that? can you please explain?



EMPOWERgmat Instructor
Status: GMAT Assassin/CoFounder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 11825
Location: United States (CA)
GRE 1: 340 Q170 V170

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
05 Dec 2017, 13:09
Hi All, This is a tougher probability question than average (and you likely will not see this exact situation on Test Day). This is meant to say that you shouldn't be too concerned about this prompt until you're picking up points in all the other 'gettable' areas first. That having been said, the 'quirk' with this question is that you have to account for the probability of rolling two dice and getting the same number on both dice vs. getting two different numbers. When rolling two dice, there are 36 possible outcomes: 6 outcomes have the same number twice (11, 22, etc.) 30 outcomes have two different numbers (14, 32, 51, etc.) Thus, 6/36 = 1/6 of the outcomes are the same number twice 30/36 = 5/6 of the outcomes are two different numbers IF Pam rolled 55, then Robin would have to also roll 55. The probability of that occurring would be: (1/6)(1/6) = 1/36. IF Pam rolled 26, then Robin has two different ways to match up (26 OR 62). The probability of that occurring would be (2/6)(1/6) = 2/36 = 1/18 Accounting for all possible outcomes, the probability of Pam and Robin rolling the same result would be: (1/6)(1/36) + (5/6)(1/18) = 1/216 + 5/108 = 1/216 + 10/216 = 11/216 Final Answer: GMAT assassins aren't born, they're made, Rich
_________________
760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com
Rich Cohen
CoFounder & GMAT Assassin
Special Offer: Save $75 + GMAT Club Tests Free
Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/
***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***********************



Intern
Joined: 10 Apr 2013
Posts: 2

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
27 Feb 2018, 06:43
EMPOWERgmatRichC wrote: Hi All, Accounting for all possible outcomes, the probability of Pam and Robin rolling the same result would be: (1/6)(1/36) + (5/6)(1/18) = 1/216 + 5/108 = 1/216 + 10/216 = 11/216
Please clarify why you included additional probabilities (highlighted in red).



EMPOWERgmat Instructor
Status: GMAT Assassin/CoFounder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 11825
Location: United States (CA)
GRE 1: 340 Q170 V170

Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr [#permalink]
Show Tags
27 Feb 2018, 10:45
xclsx wrote: EMPOWERgmatRichC wrote: Hi All, Accounting for all possible outcomes, the probability of Pam and Robin rolling the same result would be: (1/6)(1/36) + (5/6)(1/18) = 1/216 + 5/108 = 1/216 + 10/216 = 11/216
Please clarify why you included additional probabilities (highlighted in red). Hi xclsx, We have to account for two possible situations (since Pam's roll dictates the probability that Robin's roll will match) : Pam rolls the same number on both dice or Pam rolls two different numbers. The probabilities that you highlighted in red are those two probabilities: there's a 1/6 chance that it's the same number twice and a 5/6 chance that it's two different numbers. GMAT assassins aren't born, they're made, Rich
_________________
760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com
Rich Cohen
CoFounder & GMAT Assassin
Special Offer: Save $75 + GMAT Club Tests Free
Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/
***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***********************




Re: Pam and Robin each roll a pair of fair, sixsided dice. What is the pr
[#permalink]
27 Feb 2018, 10:45






