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A bag contains ten marbles of the same size: 3 are identical green mar

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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  [#permalink]

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New post 29 Jan 2015, 10:48
3
25
00:00
A
B
C
D
E

Difficulty:

  95% (hard)

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) 82


Counting & combination problems on the GMAT are tricky because many of them don't fit into nice neat formulas. They require some efficient out-of-the-box 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- ... -the-gmat/

Mike :-)

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Re: A bag contains ten marbles of the same size: 3 are identical green mar  [#permalink]

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New post 30 Jan 2015, 04:53
1
1
2
hi..
there will be different cases, which we will take one by one..
1)case 1 when all five colours are different-7c5=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...
total-82 ans E
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Re: A bag contains ten marbles of the same size: 3 are identical green mar  [#permalink]

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New post 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  [#permalink]

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New post 01 Feb 2015, 17:49
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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 akhil911
My 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 700-level 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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Re: A bag contains ten marbles of the same size: 3 are identical green mar  [#permalink]

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New post 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  [#permalink]

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New post 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 5-10 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 time-consuming 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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Re: A bag contains ten marbles of the same size: 3 are identical green mar  [#permalink]

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New post 04 Mar 2015, 05:50
1
1
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  [#permalink]

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New post 15 Mar 2015, 04:58
2
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) 82


Counting & combination problems on the GMAT are tricky because many of them don't fit into nice neat formulas. They require some efficient out-of-the-box 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- ... -the-gmat/

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  [#permalink]

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New post 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 5-10 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 time-consuming 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  [#permalink]

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New post 23 Mar 2015, 12:00
1
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 right-brain skill, a kind of pattern-matching that can't be summarized in a step-by-step 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  [#permalink]

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New post 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  [#permalink]

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New post 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  [#permalink]

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New post 05 Jun 2017, 12:51
1
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 PMZ21

I'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  [#permalink]

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New post 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  [#permalink]

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New post 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 different-7c5=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...
total-82 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  [#permalink]

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New post 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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A bag contains ten marbles of the same size: 3 are identical green mar &nbs [#permalink] 02 Oct 2018, 21:29
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