Author 
Message 
TAGS:

Hide Tags

SVP
Joined: 21 Jul 2006
Posts: 1510

Out of seven models, all of different heights, five models [#permalink]
Show Tags
30 Nov 2007, 08:54
6
This post received KUDOS
20
This post was BOOKMARKED
Question Stats:
53% (03:13) correct
47% (02:24) wrong based on 378 sessions
HideShow timer Statistics
Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible? A. 6 B. 11 C. 17 D. 72 E. 210
Official Answer and Stats are available only to registered users. Register/ Login.



Senior Manager
Joined: 09 Oct 2007
Posts: 466

5
This post received KUDOS
I got 17, is it correct?
Ways in which you can sit all models in oder: 7!/5!2! = 21
Then we figure out in how many of those 21 options model 4 and 6 are sitting together.
I usually draw something like this:
Option 1: _ _ _ _ _ Model 4, 6 could take this spaces, leaving last spot for model 7. You can have 3 in which you can arrange the first 3 models in the first 2 spots = 3
Option 2: _ _ _ _ _ Model 4, 6 could also be on the last 2 spots, but there's only one option on this one, because there are only 3 models to fill the first 3 spots = 1
21  (3+1) = 17



Manager
Joined: 01 Sep 2007
Posts: 100
Location: Astana

Re: PS: Permutations & Combinations [#permalink]
Show Tags
22 Dec 2007, 10:45
tarek99 wrote: If the five models are to stand in a line from shortest to tallest
I feel this condition is irrelevant because in the end the problem asks for the number of ways models can be arranged so that bla bla. It got me confused because in the begining I thought models should stand in ascending order on a photograph =) narrowing down possible ways of arranging them to fewer than 9. what is the source of the problem?



CEO
Joined: 29 Mar 2007
Posts: 2559

Re: PS: Permutations & Combinations [#permalink]
Show Tags
22 Dec 2007, 17:45
1
This post was BOOKMARKED
CaspAreaGuy wrote: tarek99 wrote: If the five models are to stand in a line from shortest to tallest I feel this condition is irrelevant because in the end the problem asks for the number of ways models can be arranged so that bla bla. It got me confused because in the begining I thought models should stand in ascending order on a photograph =) narrowing down possible ways of arranging them to fewer than 9. what is the source of the problem?
I agree this messed part up as well.
7!/5!*2! = 21 ways
12346 No
12467 No
23467 No
13467 No
so 17 ways.



Director
Joined: 03 Sep 2006
Posts: 871

Re: PS: Permutations & Combinations [#permalink]
Show Tags
22 Dec 2007, 18:09
tarek99 wrote: Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible?
a) 6 b) 11 c) 17 d) 72 e) 210
Please show your steps
Say we have the models numbered 1~7 ( assume, 1 is shortest and 7 is tallest in that order )
1, 2, 3, 4, 5, 6, 7.
Question says, 4th and 6th can't be adjacent, which implies question "implies" 4th and 6th are always selected amongst the 5 models but never sit/stand together.
(4,6)XXX
The remaining three can be selected in 5C3 ways, and (4 and 6 ) can arrange amongst themselves in 2! ways.
Therefore number of ways choosing,5 models so that 4 and 6 are akways included are
5C3*2! = 20
This also includes the number of ways in which(4,6) are together.
Then, the number of ways in which (4,6) will be always together can be computed
If (4,6) occupy any of the two adjacent places, then remaining three places can be occupied by (xxx) in 3! ways.
Or
46xxx
x46xx
xx46x
Thus answer is
(2!*5C3)3! = 17



Manager
Joined: 01 Sep 2007
Posts: 100
Location: Astana

Re: PS: Permutations & Combinations [#permalink]
Show Tags
22 Dec 2007, 23:37
GMATBLACKBELT wrote: CaspAreaGuy wrote: tarek99 wrote: If the five models are to stand in a line from shortest to tallest I feel this condition is irrelevant because in the end the problem asks for the number of ways models can be arranged so that bla bla. It got me confused because in the begining I thought models should stand in ascending order on a photograph =) narrowing down possible ways of arranging them to fewer than 9. what is the source of the problem? I agree this messed part up as well. 7!/5!*2! = 21 ways 12346 No 12467 No 23467 No 13467 No so 17 ways.
Ok, agreed. They do stand in ascending order. Therefore, 7!/(5!2!)4 = 17



Director
Joined: 25 Oct 2006
Posts: 635

Re: PS: Permutations & Combinations [#permalink]
Show Tags
14 Aug 2008, 13:15
5
This post received KUDOS
1
This post was BOOKMARKED
Here is how I think... 7 models say 1,2,3,4,5,6,7 Now find out the arrangements where 4 and 6 are NOT adjacent or seat together = total arrangements  Seat together total arrangements = 7C5 = 21 Seat together = If assume that 4 and 6 are selected in the pool already, need to find out other three members. Therefore we have 73 = 4 members left. I deduct 3 because we have to neglect 5 also otherwise 4 and 6 can't be adjacent (the five models are to stand in a line from shortest to tallest  here is the significance). Now select 3 people from 4 members in 4C3 ways. Lets form the equation: the arrangements where 4 and 6 are NOT adjacent or stand together = 7C5  4C3 = 21  4 = 17.
_________________
If You're Not Living On The Edge, You're Taking Up Too Much Space



Manager
Joined: 05 Jun 2009
Posts: 110

Re: PS: Permutations & Combinations [#permalink]
Show Tags
05 Sep 2009, 10:32
3
This post received KUDOS
1
This post was BOOKMARKED
lets say 1 2 3 4 5 6 7 are models with 1 to 7 from smallest to tallest. no of ways to slect any 5 models from 7 models =7C5 =21 no of arrangement for any selection would be =1 there for no of arrangement for 21 selection =21 now 4 and 6 should not be adjusant ,so lets subtract the cases in which 4 and 6 are adjusant. 4 and 6 can be adjusant only when 5 is not selected . so we have decided about selecting two models 4 & 6 and not selecting 5 . no of models left (1 2 3 4 5 6 7) (4,6 )5 =1 2 3 7 only four models are left for selection so to make a group of 5 models we have to select 3 out of 4 (or drop 1 out of 4) =4C3 (or 4C1)=4 214=17



Manager
Joined: 27 Oct 2008
Posts: 185

Re: PS: Permutations & Combinations [#permalink]
Show Tags
27 Sep 2009, 22:31
3
This post received KUDOS
Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible?
a) 6 b) 11 c) 17 d) 72 e) 210
Soln: Total number of ways of choosing 5 out of 7 models is = 7C5 Number of combinations where 4th tallest and 6th tallest height models will be chosen is = 5C3 Of these combinations of 4th tallest and 6th tallest, the combinations in which 4th and 6th will be chosen but will not come together when arranged in increasing order of height is = 4C2
= 7C5  5C3 + 4C2 = 21  10 + 6 = 17
Ans is B



Manager
Joined: 24 Jul 2009
Posts: 73
Location: United States

Re: PS: Permutations & Combinations [#permalink]
Show Tags
14 Nov 2009, 23:24
kudos srivas, u made it sound simple....... I was actually planning to post this problem, thank god i searched for it before......



Senior Manager
Joined: 22 Dec 2009
Posts: 359

Re: PS: Permutations & Combinations [#permalink]
Show Tags
16 Feb 2010, 12:51
srivas wrote: Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible?
a) 6 b) 11 c) 17 d) 72 e) 210
Soln: Total number of ways of choosing 5 out of 7 models is = 7C5 Number of combinations where 4th tallest and 6th tallest height models will be chosen is = 5C3 Of these combinations of 4th tallest and 6th tallest, the combinations in which 4th and 6th will be chosen but will not come together when arranged in increasing order of height is = 4C2
= 7C5  5C3 + 4C2 = 21  10 + 6 = 17
Ans is B Could someone explain me the part highlighted in red? Thanks
_________________
Cheers! JT........... If u like my post..... payback in Kudos!!
Do not post questions with OAPlease underline your SC questions while postingTry posting the explanation along with your answer choice For CR refer Powerscore CR BibleFor SC refer Manhattan SC Guide
~~Better Burn Out... Than Fade Away~~



Math Expert
Joined: 02 Sep 2009
Posts: 39719

Re: PS: Permutations & Combinations [#permalink]
Show Tags
16 Feb 2010, 13:22
2
This post received KUDOS
Expert's post
4
This post was BOOKMARKED
jeeteshsingh wrote: srivas wrote: Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible?
a) 6 b) 11 c) 17 d) 72 e) 210
Soln: Total number of ways of choosing 5 out of 7 models is = 7C5 Number of combinations where 4th tallest and 6th tallest height models will be chosen is = 5C3 Of these combinations of 4th tallest and 6th tallest, the combinations in which 4th and 6th will be chosen but will not come together when arranged in increasing order of height is = 4C2
= 7C5  5C3 + 4C2 = 21  10 + 6 = 17
Ans is B Could someone explain me the part highlighted in red? Thanks When we choose 4 and 6 and they are not adjacent means that we must choose 5 too (to stand between them). So for this case we must choose 4, 5, and 6 (3C3) and 2 other from 4 left (4C2) = 3C3*4C2=4C2. This can be solved in another way: If we choose 4 and 6, we must also choose 5 (to stand between them) =3C3*4C2=4C2=6 We can choose either 4 or 6 = 2*1C1*5C4=10 We can choose neither 4 nor 6 = 5C5=1 6+10+1=17. Answer: C (17). Hope it helps.
_________________
New to the Math Forum? Please read this: 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



Senior Manager
Joined: 22 Dec 2009
Posts: 359

Re: PS: Permutations & Combinations [#permalink]
Show Tags
16 Feb 2010, 14:11
Bunuel wrote: jeeteshsingh wrote: srivas wrote: Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible?
a) 6 b) 11 c) 17 d) 72 e) 210
Soln: Total number of ways of choosing 5 out of 7 models is = 7C5 Number of combinations where 4th tallest and 6th tallest height models will be chosen is = 5C3 Of these combinations of 4th tallest and 6th tallest, the combinations in which 4th and 6th will be chosen but will not come together when arranged in increasing order of height is = 4C2
= 7C5  5C3 + 4C2 = 21  10 + 6 = 17
Ans is B Could someone explain me the part highlighted in red? Thanks When we choose 4 and 6 and they are not adjacent means that we must choose 5 too (to stand between them). So for this case we must choose 4, 5, and 6 (3C3) and 2 other from 4 left (4C2) = 3C3*4C2=4C2. This can be solved in another way: If we choose 4 and 6, we must also choose 5 (to stand between them) =3C3*4C2=4C2=6 We can choose either 4 or 6 = 2*1C1*5C4=10 We can choose neither 4 nor 6 = 5C5=1 6+10+1=17. Answer: C (17). Hope it helps. Thanks Bunuel... I missed "are to stand in a line from shortest to tallest"... and hence I was expecting arrangements like x4xx6... too!... My bad
_________________
Cheers! JT........... If u like my post..... payback in Kudos!!
Do not post questions with OAPlease underline your SC questions while postingTry posting the explanation along with your answer choice For CR refer Powerscore CR BibleFor SC refer Manhattan SC Guide
~~Better Burn Out... Than Fade Away~~



GMAT Club Legend
Joined: 09 Sep 2013
Posts: 16002

Re: Out of seven models, all of different heights, five models [#permalink]
Show Tags
06 Jan 2014, 20:46
Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up  doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources



Intern
Joined: 02 Oct 2013
Posts: 12

Re: Out of seven models, all of different heights, five models [#permalink]
Show Tags
07 Jan 2014, 12:48
2
This post received KUDOS
So 7C5 gives us total number of combinations =21
We need to subtract the number of ways that the 4th and 6th can be next to each other.
The 4th and 6th tallest will only be next to each other when the 5th is NOT selected. The number of combinations where the 5th is not selected is 4  (two not selected from 7, one of them is the 5th, the others can be 1st, 2nd, 3rd and 7th.)
So we have 214 = 17 ways.



GMAT Club Legend
Joined: 09 Sep 2013
Posts: 16002

Re: Out of seven models, all of different heights, five models [#permalink]
Show Tags
10 Jul 2015, 19:28
Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up  doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources



Manager
Joined: 14 Jul 2014
Posts: 191
Location: United States
GMAT 1: 600 Q48 V27 GMAT 2: 720 Q50 V37
GPA: 3.2

Re: Out of seven models, all of different heights, five models [#permalink]
Show Tags
08 Feb 2016, 09:04
Seven models in order: ABCDEFG. D and F should not be adjacent to each other.
Total arrangements: 7C5 = 21.
When D and F are next to each other: ABCDF ABDFG BCDFG ACDFG
so 214 = 17 ways.



GMAT Club Legend
Joined: 09 Sep 2013
Posts: 16002

Re: Out of seven models, all of different heights, five models [#permalink]
Show Tags
09 Feb 2017, 23:25
Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up  doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources



Manager
Joined: 03 Apr 2013
Posts: 179

Re: Out of seven models, all of different heights, five models [#permalink]
Show Tags
16 Jun 2017, 23:41
tarek99 wrote: Out of seven models, all of different heights, five models will be chosen to pose for a photograph. If the five models are to stand in a line from shortest to tallest, and the fourthtallest and sixthtallest models cannot be adjacent, how many different arrangements of five models are possible?
A. 6 B. 11 C. 17 D. 72 E. 210 Okay..here's my take.. Let the models be(in ascending order of height) A B C D E F G Now..out of these 7..we have to select 5 and make them stand in ascending order of height. According to the question, B and D cannot be standing together. Imagine this, if we select any 5 of these, there will only be one way to make them stand. Considering the situation, if both B and D were selected, and let's say that B and D were actually selected in the group; then they will stand together every time C is not the part of the group(as there will be no one to stand in the middle). So, our complement event is that B and D are selected, and C is not considered at all for selection. In this way, they will always stand together. Number of ways to do this.. B and D are already in the group, so we have to select the remaining 3 out of 5. But wait, we also have to never consider C for selection. Finally then, we have to select the remaining 3 out of 4(where C is excluded) \(4C3 = 4\) This has to be subtracted from the total possible combinations. \(7C5  4 = 17\) Answer (C)
_________________
Spread some love..Like = +1 Kudos




Re: Out of seven models, all of different heights, five models
[#permalink]
16 Jun 2017, 23:41







