Author 
Message 
TAGS:

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 46284

Question Stats:
57% (01:24) correct 43% (02:38) wrong based on 60 sessions
HideShow timer Statistics



Math Expert
Joined: 02 Sep 2009
Posts: 46284

Re M2827 [#permalink]
Show Tags
16 Sep 2014, 01:29
Official Solution:Certain bowl contains 5 red marbles and 3 blue marbles only. One by one, every marble is drawn at random and without replacement. What is the probability that the seventh marble drawn is NOT blue? A. \(\frac{7}{8}\) B. \(\frac{3}{4}\) C. \(\frac{2}{3}\) D. \(\frac{5}{8}\) E. \(\frac{3}{8}\) Basically we need to find the probability that the seventh marble drawn is red (so not blue). Now, the initial probability of drawing red marble is \(\frac{5}{8}\). Without knowing the other results, the probability of drawing red marble will not change for ANY successive draw: second, third, fourth, ..., seventh. Thus the probability that the seventh marble is red is \(\frac{5}{8}\). The same for blue marble: the probability of drawing blue marble is \(\frac{3}{8}\), the probability that for instance the 8th marble drawn is blue is still \(\frac{3}{8}\). There is simply no reason to believe WHY is any draw different from another (provided we don't know the other results). Answer: D
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Intern
Joined: 11 Nov 2014
Posts: 1

Re: M2827 [#permalink]
Show Tags
13 Nov 2014, 17:36
Hi there, A new member here, just joined a day or two back I'm quite confused about the solution to this problem. How is it possible that the probability remains constant even when we are removing marbles without replacement? Wouldn't the denominators in the probabilities adjust for each draw? Any help would be greatly appreciated! Thanks, Francis



Math Expert
Joined: 02 Sep 2009
Posts: 46284

Re: M2827 [#permalink]
Show Tags
14 Nov 2014, 02:52
francisdarby wrote: Hi there, A new member here, just joined a day or two back I'm quite confused about the solution to this problem. How is it possible that the probability remains constant even when we are removing marbles without replacement? Wouldn't the denominators in the probabilities adjust for each draw? Any help would be greatly appreciated! Thanks, Francis Consider this: there are 8 cards, 5 spades, 3 hearts. What is the probability that the first card you pick will be spade? Clearly 5/8. Now next question, suppose I throw away three cards at random, not telling you which cards I thrown away. What is the probability NOW that you pick a spade out of 5 remaining cards? I've just reduced the sample from which you pick, but does the probability changed? WHY should it change? Another question, the same cards 5 spades and 3 hearts: I say that you can pick one card but only out of randomly selected 5 cards. What is the probability that the card you pick will be spade? There is no difference in ALL above cases and the probability will remain 5/8. If it changed, increased (or decreased), it would mean that the probability of picking heart on the other hand decreased (or increased). Why would it? Similar questions to practice: aboxcontains3yellowballsand5blackballsonebyone90272.htmlabagcontains3whiteballs3blackballs2redballs100023.htmleachoffourdifferentlockshasamatchingkeythekeys101553.htmlif40peoplegetthechancetopickacardfromacanister97015.htmlabagcontains3whiteballs3blackballs2redballs100023.htmlHope this helps.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Intern
Joined: 24 Nov 2014
Posts: 18

Re: M2827 [#permalink]
Show Tags
19 Mar 2015, 14:01
But if i ask the question what is P that the 2nd marble is red, wouldnt it be like that: P(b,r)+P(r,r) = 5/8*3/7 + 3/8*2/7 = 3/8
So how can P that the 7th marble is red be 5/8? How can it be for all the same if there is NO replacement?



Math Expert
Joined: 02 Sep 2009
Posts: 46284

Re: M2827 [#permalink]
Show Tags
20 Mar 2015, 05:58



Intern
Joined: 29 Jun 2016
Posts: 11

Re: M2827 [#permalink]
Show Tags
18 Aug 2016, 10:40
Hi Bunuel,
My understanding about this problem is that this is a case of a nonmutually exclusive event, as the execution of one event affects another, below is written what I understand from this problem.
If there are 8 marbles, and if, someone drew marbles in the manner below without replacement; B B R R R R R R
Then the respective probabilities of drawing at succession are; 3/8 2/7 5/6 4/5 3/4 2/3 1/2 1
See that above drawing leaves only a red at the seventh draw, but to reach to the seventh draw event we should also change the sample space along with the no. of favorable cases.
Do you agree?



Intern
Joined: 29 Jun 2016
Posts: 11

Re: M2827 [#permalink]
Show Tags
18 Aug 2016, 10:41
There will be more such cases to leave a nonblue at 7th draw we just need to add those cases



Math Expert
Joined: 02 Sep 2009
Posts: 46284

Re: M2827 [#permalink]
Show Tags
18 Aug 2016, 10:52
subhajit1 wrote: Hi Bunuel,
My understanding about this problem is that this is a case of a nonmutually exclusive event, as the execution of one event affects another, below is written what I understand from this problem.
If there are 8 marbles, and if, someone drew marbles in the manner below without replacement; B B R R R R R R
Then the respective probabilities of drawing at succession are; 3/8 2/7 5/6 4/5 3/4 2/3 1/2 1
See that above drawing leaves only a red at the seventh draw, but to reach to the seventh draw event we should also change the sample space along with the no. of favorable cases.
Do you agree? Unfortunately your understanding is wrong. I suggest you to follow the links given above.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Manager
Joined: 23 Nov 2016
Posts: 76
Location: United States (MN)
GPA: 3.51

Re: M2827 [#permalink]
Show Tags
09 Feb 2017, 18:10
madmax1000 wrote: But if i ask the question what is P that the 2nd marble is red, wouldnt it be like that: P(b,r)+P(r,r) = 5/8*3/7 + 3/8*2/7 = 3/8
So how can P that the 7th marble is red be 5/8? How can it be for all the same if there is NO replacement? The reason why you got 3/8 is because you found the probability of the second draw being blue. It should be: P(b,r)+P(r,r) = 3/8*5/7 + 5/8*4/7= 5/8 Heavily recommend drawing a probability tree (try excel) to see how the probability doesn't change up until you have no more draws left. After 8 draws, the probability then drops down to zero for either color .



Senior Manager
Status: You have to have the darkness for the dawn to come
Joined: 09 Nov 2012
Posts: 315
Daboo: Sonu
GMAT 1: 590 Q49 V20 GMAT 2: 730 Q50 V38

Re: M2827 [#permalink]
Show Tags
24 Mar 2017, 11:13
Bunuel wrote: Official Solution:
Certain bowl contains 5 red marbles and 3 blue marbles only. One by one, every marble is drawn at random and without replacement. What is the probability that the seventh marble drawn is NOT blue?
A. \(\frac{7}{8}\) B. \(\frac{3}{4}\) C. \(\frac{2}{3}\) D. \(\frac{5}{8}\) E. \(\frac{3}{8}\)
Basically we need to find the probability that the seventh marble drawn is red (so not blue). Now, the initial probability of drawing red marble is \(\frac{5}{8}\). Without knowing the other results, the probability of drawing red marble will not change for ANY successive draw: second, third, fourth, ..., seventh. Thus the probability that the seventh marble is red is \(\frac{5}{8}\). The same for blue marble: the probability of drawing blue marble is \(\frac{3}{8}\), the probability that for instance the 8th marble drawn is blue is still \(\frac{3}{8}\). There is simply no reason to believe WHY is any draw different from another (provided we don't know the other results).
Answer: D Hi Bunuel the question says without replacement. It means after each draws the no of ball reduces by 1. so how the probability of drawing 7th ball remains same as probability of drawing 1st ball red
_________________
You have to have the darkness for the dawn to come.
Give Kudos if you like my post



Math Expert
Joined: 02 Sep 2009
Posts: 46284

Re: M2827 [#permalink]
Show Tags
24 Mar 2017, 11:16
daboo343 wrote: Bunuel wrote: Official Solution:
Certain bowl contains 5 red marbles and 3 blue marbles only. One by one, every marble is drawn at random and without replacement. What is the probability that the seventh marble drawn is NOT blue?
A. \(\frac{7}{8}\) B. \(\frac{3}{4}\) C. \(\frac{2}{3}\) D. \(\frac{5}{8}\) E. \(\frac{3}{8}\)
Basically we need to find the probability that the seventh marble drawn is red (so not blue). Now, the initial probability of drawing red marble is \(\frac{5}{8}\). Without knowing the other results, the probability of drawing red marble will not change for ANY successive draw: second, third, fourth, ..., seventh. Thus the probability that the seventh marble is red is \(\frac{5}{8}\). The same for blue marble: the probability of drawing blue marble is \(\frac{3}{8}\), the probability that for instance the 8th marble drawn is blue is still \(\frac{3}{8}\). There is simply no reason to believe WHY is any draw different from another (provided we don't know the other results).
Answer: D Hi Bunuel the question says without replacement. It means after each draws the no of ball reduces by 1. so how the probability of drawing 7th ball remains same as probability of drawing 1st ball red I tried to explain this here: https://gmatclub.com/forum/m28184525.html#p1442558
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Intern
Joined: 09 Jul 2016
Posts: 17

Re M2827 [#permalink]
Show Tags
31 May 2017, 09:59
This is a very good questions and hits right on the basics.



Intern
Joined: 06 Apr 2017
Posts: 29
Location: United States (OR)
Concentration: Finance, Leadership
GMAT 1: 730 Q48 V44 GMAT 2: 730 Q49 V40
GPA: 3.98
WE: Corporate Finance (Health Care)

Re: M2827 [#permalink]
Show Tags
13 Jul 2017, 10:11
For those of you who are confused by Bunuel's beautiful answer, you can arrive at the same solution using permutations.
Let's find the possible permutations for the first 6 balls, and then worry about the 7th and 8th after that
5 Reb and 1 Blue  We are arranging 6 balls of two different types. To arrange 6 objects without respect for repetition of each type, the formula is simply \(6!\). To account for repeating types, reduce \(6!\) by the factorial of the repetition number for each type.
\(\frac{6!}{5!1!}=6\)
4 Red and 2 Blue
\(\frac{6!}{4!2!}15\)
3 Red and 3 Blue
\(\frac{6!}{3!3!}=20\)
Now multiply each by the available remaining permutations of the 7th and 8th ball:
5 Reb and 1 Blue?
Only BB is left, so \(6*1=6\)
4 Reb and 2 Blue?
RB and BR are left, so \(15*2=30\)
3 Reb and 3 Blue?
Only RR is left, so \(20*1=20\)
Thus, there are \(6+30+20=56\) total arrangements
In how many of these last three options are the 7th ball red?
5 Red and 1 Blue: \(0\) 4 Red and 2 Blue: \(15\) 3 Red and 3 Blue: \(20\)
\(\frac{35}{56}=\frac{5}{8}\)
Answer D



Intern
Joined: 11 Oct 2017
Posts: 11

Re: M2827 [#permalink]
Show Tags
13 Jan 2018, 07:06
A simple answer using combinatorics
if the 7th slot is NOT BLUE, it is RED now ignore this slot completely. it will not affect the other arrangements. for the 7 remaining slots, we have 7 marbles = (7! arrangements) but since 4 Red repeats and 3 blue repeats, the possible arrangements = 7!/(3!4!) = 35 total arrangements without the restriction for the 7th slot = 8!(5!3!) = 56 desired probability = 35/56 = 5/8



Intern
Joined: 02 Jun 2013
Posts: 1

Re M2827 [#permalink]
Show Tags
17 Feb 2018, 22:58
I think this is a highquality question. I have an alternate solution, 8!/(5!*3!)  Total # of permutations  56 (i)
Of these fixing the 7th position as red  # of permutations is 7!/(4!*3!)  35(ii)
(ii)/(i) is what we want = (5/8)



Manager
Joined: 26 Feb 2018
Posts: 79
Location: United Arab Emirates
GMAT 1: 710 Q47 V41 GMAT 2: 770 Q49 V47

Re: M2827 [#permalink]
Show Tags
10 Mar 2018, 15:49
This is a simple problem disguised as something a lot more difficult and it's easy to waste a lot of time considering permutations
Consider it like this  you draw the marbles 1 at a time, therefore you have 8 positions. In all the permutations, in any one position, the probability of that marble being red is 5/8. We are considering position 7, and the probability of the marble being red is the same as all the other positions  5/8



Intern
Joined: 23 May 2017
Posts: 2

Re M2827 [#permalink]
Show Tags
25 Mar 2018, 07:09
If there was a replacement of marbles, then we could say that all draws will have the same probability. Not understood why every successive draw has the same probability.










