December 13, 2018 December 13, 2018 08:00 AM PST 09:00 AM PST What people who reach the high 700's do differently? We're going to share insights, tips and strategies from data we collected on over 50,000 students who used examPAL. December 14, 2018 December 14, 2018 09:00 AM PST 10:00 AM PST 10 Questions will be posted on the forum and we will post a reply in this Topic with a link to each question. There are prizes for the winners.
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Magoosh GMAT Instructor
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A bag contains ten marbles of the same size: 3 are identical green mar
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29 Jan 2015, 10:48
Question Stats:
39% (03:45) correct 61% (02:46) wrong based on 195 sessions
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A bag contains ten marbles of the same size: 3 are identical green marbles, 2 are identical red marbles, and the other 5 are five distinct colors. If 5 marbles are selected at random, how many distinct combinations of five marbles could be drawn? (A) 41 (B) 51 (C) 62 (D) 72 (E) 82Counting & combination problems on the GMAT are tricky because many of them don't fit into nice neat formulas. They require some efficient outofthebox thinking, as this problem does. For a set of challenging counting problems, as well as for the OE of this particular problem, see: https://magoosh.com/gmat/2015/counting ... thegmat/Mike
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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30 Jan 2015, 04:53
hi.. there will be different cases, which we will take one by one.. 1)case 1 when all five colours are different7c5=21.. 2)case 2 when three different colours and 2 of same type=6c3*2=30.. 3)case 3 when two different colours and 3 of same type=6c2=15.. 4)case 4 when one different colours and 2 of two different colors=5c1*4!/2!2!=15... 5)case 4 when 3 of one type and 2 of other colour=1... total82 ans E
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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01 Feb 2015, 06:22
Hi Mike, I have seen the solution on the Magoosh website but find that it is a very cumbersome solution. Is there any shorter way to solve this problem ???
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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01 Feb 2015, 17:49
akhil911 wrote: Hi Mike, I have seen the solution on the Magoosh website but find that it is a very cumbersome solution. Is there any shorter way to solve this problem ??? Dear akhil911My friend, I'm not sure you realize the inherent problem with the question you were asking. You are an intelligent individual, so I believe if you fully appreciated what the question implied, you wouldn't have asked it this way. I and all the other GMAT experts from other prep companies  we are all educators. We are concerned with teaching, with communicating material as efficiently as possible. If we write solutions to a problem, we are concerned with showing clearest and most efficient solutions. We would have absolutely no motivation to show a complicated solution if a much simply one were possible. Therefore, do you see the only possible implication if the solution one of us shows is, in fact, very complicated? Since we are educators striving to make everything as clear and as brief as possible, and since the solution shown, for example, in this problem, is not simple, don't you see how this inescapably has to reflect something about the nature of the mathematics itself? That problem is, in some ways, representative of a 700level Quant question. There is no simple formula one can use, just plug two or three things in a done. One has to map out a plan of attach to move through the problem. In fact, in the solution, I demonstrated a systematic way to move through the cases: if you were adept in your analysis, you could move through that entire solution in under 1 min. Producing and executing that kind of analysis: that's precisely the kind of creativity and mental flexibility you need to have if you want to cross the 700 threshold. Does all this make sense? Mike
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Schools: Booth '17, Sloan '17, CBS '17, Ross '17 (S), Haas '17, Stern '17, Yale '17, Anderson '17, Kelley '17 (S), KenanFlagler '17 (S), HEC Dec '17, HKUST '16, Jones '16, NUS '17
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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01 Feb 2015, 21:14
Hi Mike, I Apologize if you found my post was not appropriate in this regard. I am one of your biggest fans and appreciate the way you teach all the fellow students. I had understood what the problem was asking and that you were trying to explain the inherent concept underlying the question. I found the post really useful for me. The only question I had regarding this question that is there a shorter way to approach such problems as it would take up quite a lot of time if we started counting in such questions on the real Gmat. I could not think of one in this case and that's the reason I approached you. Kindly accept my Apologies. Regards Akhil
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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02 Feb 2015, 10:06
akhil911 wrote: Hi Mike,
I Apologize if you found my post was not appropriate in this regard. I am one of your biggest fans and appreciate the way you teach all the fellow students. I had understood what the problem was asking and that you were trying to explain the inherent concept underlying the question. I found the post really useful for me. The only question I had regarding this question that is there a shorter way to approach such problems as it would take up quite a lot of time if we started counting in such questions on the real Gmat. I could not think of one in this case and that's the reason I approached you. Kindly accept my Apologies.
Regards Akhil Dear Akhil, Thank you for your kind words. I took no offense. I guess I would say  understand that the GMAT will never expect you to do some kind of laborious counting that would take 510 minutes. BUT, the GMAT may expect you to dissect a problem quickly, and realize that a sum can be assembled relatively quickly from simple separate cases. I realize if this way of thinking is new to you, it may seem like a cumbersome and timeconsuming approach, but what I am suggesting is that, with practice, one can master this kind of thinking and get fast at it. Does this make sense? Mike
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Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)



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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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04 Mar 2015, 05:50
So we have GGGRRABCDE and we have to select five such that all the combinations are distinct: Let us say X denotes distinct then we have the following combinations: GGGXX, GGXXX, GXXXX, XXXXX GGGXX = GGGXX + GGGRR GGXXX = GGXXX + GGRRX GXXXX = GXXXX + GRRXX XXXXX = RRXXX + XXXXX GGGXX = 1 x (6C2) = 15 GGGRR = 1 way GGXXX = 1 x 6C3 = 20 GGRRX = 5 GXXXX = 6C4 = 15 GRRXX = 5C2 = 10 RRXXX = 5C3 = 10 XXXXX = 6C5 = 6 Total number of ways = 82
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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15 Mar 2015, 04:58
mikemcgarry wrote: A bag contains ten marbles of the same size: 3 are identical green marbles, 2 are identical red marbles, and the other 5 are five distinct colors. If 5 marbles are selected at random, how many distinct combinations of five marbles could be drawn? (A) 41 (B) 51 (C) 62 (D) 72 (E) 82Counting & combination problems on the GMAT are tricky because many of them don't fit into nice neat formulas. They require some efficient outofthebox thinking, as this problem does. For a set of challenging counting problems, as well as for the OE of this particular problem, see: https://magoosh.com/gmat/2015/counting ... thegmat/Mike 3G , 2R , and 5 Distinct. All different > C(7,5) = 21 2 same and 3 diff > C(2,1)*C(6,3) = 40 3 same and 2 diff > C(6,2) = 15 1 from distinct set + 2 green + 2 red > C(5,1) = 5 3 green & 2 red > 1 total = 82. good question! :D
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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22 Mar 2015, 04:45
mikemcgarry wrote: akhil911 wrote: Hi Mike,
I Apologize if you found my post was not appropriate in this regard. I am one of your biggest fans and appreciate the way you teach all the fellow students. I had understood what the problem was asking and that you were trying to explain the inherent concept underlying the question. I found the post really useful for me. The only question I had regarding this question that is there a shorter way to approach such problems as it would take up quite a lot of time if we started counting in such questions on the real Gmat. I could not think of one in this case and that's the reason I approached you. Kindly accept my Apologies.
Regards Akhil Dear Akhil, Thank you for your kind words. I took no offense. I guess I would say  understand that the GMAT will never expect you to do some kind of laborious counting that would take 510 minutes. BUT, the GMAT may expect you to dissect a problem quickly, and realize that a sum can be assembled relatively quickly from simple separate cases. I realize if this way of thinking is new to you, it may seem like a cumbersome and timeconsuming approach, but what I am suggesting is that, with practice, one can master this kind of thinking and get fast at it. Does this make sense? Mike Hey Mike, Thanks for the question! I think I'm pretty good at the quant section, but combinatorics really stifle me! I can't seem to identify the technique that I have to use. I understand the explanations completely, but they're things I can never come up with on my own. What do you recommend in this case? I'm very intimidated by Counting problems; Is there a way around this? Is there a source that outlines different methods to be applied in different cases? Please help! Thanks a lot!



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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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23 Mar 2015, 12:00
tgubbay1 wrote: Hey Mike,
Thanks for the question! I think I'm pretty good at the quant section, but combinatorics really stifle me! I can't seem to identify the technique that I have to use. I understand the explanations completely, but they're things I can never come up with on my own.
What do you recommend in this case? I'm very intimidated by Counting problems; Is there a way around this? Is there a source that outlines different methods to be applied in different cases? Please help!
Thanks a lot! Dear tgubbay1, I'm happy to respond. Counting problems, combinatorics, are not really about techniques. Much more important is the ability to frame the problem correctly, and this is a rightbrain skill, a kind of patternmatching that can't be summarized in a stepbystep way. See the discussion in this blog article: http://magoosh.com/gmat/2013/difficult ... problems/Mike
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A bag contains ten marbles of the same size: 3 are identical green mar
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28 May 2017, 00:55
DDDDD = 1 RDDDD > 5C4 = 5 GDDDD > 5C4 = 5 RRDDD > 5C3 = 10 GGDDD > 5C3 = 10 GGGDD > 5C2 = 10 RGDDD > 5C3 = 10 RGGDD > 5C2 = 10 RGGGD > 5C1 = 5 RRGDD > 5C2 = 10 RRGGD > 5C1 = 5 RRGGG = 1
Combinations are combinations of D. Hence total is 82 combinations.



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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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05 Jun 2017, 08:05
Hi Mike,
Please correct me if I'm wrong, as I'm slightly confused with what the question stem is asking.
When the question states  " If 5 marbles are selected at random, how many distinct combinations of five marbles could be drawn?" I inferred it as how many such combinations can we get, where each of the marbles chosen are distinct (by color, ofc).
And so my solution was: 5C5 + 5C3 (w/ one green and one red) + 5C4*2 (4 distinct with one green + 4 distinct with one red) = 21 Could you please tell me where have I gone wrong? I have gone through your solution in the blog post. However, that is taking into consideration 2 greens etc  which would mean that the 5 chosen marbles are not distinct. Please help!



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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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05 Jun 2017, 12:51
PMZ21 wrote: Hi Mike,
Please correct me if I'm wrong, as I'm slightly confused with what the question stem is asking.
When the question states  " If 5 marbles are selected at random, how many distinct combinations of five marbles could be drawn?" I inferred it as how many such combinations can we get, where each of the marbles chosen are distinct (by color, ofc).
And so my solution was: 5C5 + 5C3 (w/ one green and one red) + 5C4*2 (4 distinct with one green + 4 distinct with one red) = 21 Could you please tell me where have I gone wrong? I have gone through your solution in the blog post. However, that is taking into consideration 2 greens etc  which would mean that the 5 chosen marbles are not distinct. Please help! Dear PMZ21I'm happy to respond. My friend, what I wrote was: . . . how many distinct combinations of five marbles . . . and what you interpreted was: . . . how many combinations of five distinct marbles . . . If you compare those two, you will see that in my question, the combinations, not the marbles are distinct. Let's say the colors are green = G red = R black = B white = W yellow = Y purple = P sky blue = S What I am saying is that {G, G, R, R, W} is a distinct combination from {G, G, R, R, P}. In other words, if we list the colors of the five marbles, that overall list of colors will be different for different combinations. It's not that each marble will be a different color. BTW, for what you were calculating, how many different sets of five in which each of the five marbles is a different color, I think you calculated it the hard way. You could just use 7C5 = 21. Does all this make sense? Mike
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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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05 Jun 2017, 14:02
Hi Mike, Perfect, yes that makes complete sense. Thanks! You and Bunuel are such a blessing Posted from my mobile device



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Re: A bag contains ten marbles of the same size: 3 are identical green mar
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17 Jun 2018, 13:24
chetan2u wrote: hi.. there will be different cases, which we will take one by one.. 1)case 1 when all five colours are different7c5=21.. 2)case 2 when three different colours and 2 of same type=6c3*2=30.. 3)case 3 when two different colours and 3 of same type=6c2=15.. 4)case 4 when one different colours and 2 of two different colors=5c1*4!/2!2!=15... 5)case 4 when 3 of one type and 2 of other colour=1... total82 ans E Sorry, would you explain a little bit more about how you calculate case 2 and case 4? because the result I got for case 2 is 40 and for case 4 is 5. Thank you



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A bag contains ten marbles of the same size: 3 are identical green mar
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02 Oct 2018, 21:29
My process was long but I hope it is a bit more easier to understand. First I found the various distinct combinations as such    Note abbreviations  > 3 Green Marbles = 3G > 2 Red Marbles = 2G > 5 Distinct Marbles = 5D
Combinations  1) No of ways of selecting 5, when 5 selected from 5D = 1 way
2) Lets consider situations when 4 are selected from the distinct colored (4D+1G) and (4D+1R) = 5C4 x 2 = 10 ways
3) No of ways of selecting 5, when 3 selected from D group (3D+2G) and (3D+2R) and (3D+1G+1R) = 5C3 x 3 = 30 ways
4) No of ways of selecting 5, when 2 selected from D group (2D+3G) and (2D+2G+1R) and (2D+1G+2R) = 5C2 x 3 = 30 ways
5) No of ways of selecting 5, when only 1 selected from D group (1D+3G+R) and (1D+2G+2R) = 5C1 x 2 = 10 ways
6) No of ways of selecting 5, when 0 selected from D group Only one way  (3G+2R) = 1 way
Total = 82 ways
Doing it this method is not really good as you can miss some cases, but it helps understand whats happening. For example, lets consider the 5 distinct colours are Violet, Indigo, Blue, Yellow, Orange and White i.e. {V,I,B,Y,O,W}
So in point 2 described above  each 4D combination can have the last marble as G or R, simply put a single combination of 4D (such as V,I,B,Y) has 2 ways to be picked either with Green or Red = {V,I,B,Y,R} or {V,I,B,Y,G}. Hence in this case, each 4D combination is multiplied by 2.
Similarly in point 3, for a 3D combination, the last 2 marbles can be picked in 3 ways, i.e for a single 3D combination the following combinations exist  {V,I,B,G,G} {V,I,B,G,R} {V,I,B,R,R} = 3 ways for {V,I,B} to be completed. Hence we multiply each 3D combination by 3.
and so on...




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