Author 
Message 
TAGS:

Hide Tags

Intern
Joined: 09 Dec 2008
Posts: 22
Location: Vietnam
Schools: Somewhere

Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
Updated on: 08 Jan 2013, 03:16
Question Stats:
37% (02:30) correct 63% (02:39) wrong based on 287 sessions
HideShow timer Statistics
Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Official Answer and Stats are available only to registered users. Register/ Login.
Originally posted by dungtd on 29 Jul 2010, 14:24.
Last edited by Bunuel on 08 Jan 2013, 03:16, edited 1 time in total.
RENAMED THE TOPIC.




Math Expert
Joined: 02 Sep 2009
Posts: 59268

Re: Voucher!!!
[#permalink]
Show Tags
29 Jul 2010, 19:42
dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! Clearly there are more than 8 vouchers as each of four can get at least 2. So, basically 120 ways vouchers can the distributed are the ways to distribute \(x8\) vouchers, so that each can get from zero to \(x8\) as at "least 2", or 2*4=8, we already booked. Let \(x8\) be \(k\). In how many ways we can distribute \(k\) identical things among 4 persons? Well there is a formula for this but it's better to understand the concept. Let \(k=5\). And imagine we want to distribute 5 vouchers among 4 persons and each can get from zero to 5, (no restrictions). Consider: \(ttttt\) We have 5 tickets (t) and 3 separators between them, to indicate who will get the tickets: \(ttttt\) Means that first nephew will get all the tickets, \(ttttt\) Means that first got 0, second 1, third 3, and fourth 1 And so on. How many permutations (arrangements) of these symbols are possible? Total of 8 symbols (5+3=8), out of which 5 \(t\)'s and 3 \(\)'s are identical, so \(\frac{8!}{5!3!}=56\). Basically it's the number of ways we can pick 3 separators out of 5+3=8: \(8C3\). So, # of ways to distribute 5 tickets among 4 people is \((5+41)C(41)=8C3\). For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\). > \(k=7\). Plus the 8 tickets we booked earlier: \(x=k+8=7+8=15\). Answer: C (15). P.S. How to solve \((k+1)(k+2)(k+3)=3!*120=720\): 720, three digit integer ending with 0, is the product of three consecutive integers. Obviously one of them must be multiple of 5: try \(5*6*7=210<720\), next possible triplet \(8*9*10=720\), OK. So \(k+1=8\) > \(k=7\). P.P.S. Direct formula: The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items is \(n+r1C_{r1}\).
The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\).Hope it helps.
_________________




Manager
Joined: 20 Mar 2010
Posts: 66

Re: Voucher!!!
[#permalink]
Show Tags
29 Jul 2010, 18:41
bogos wrote: It took me 10 minutes to solve it First, it is remarked that all vouchers are equivalent. Let's call nephews A, B, C, D and their number of vouchers (2+a), (2+b), (2+c), and (2+d), respectively, where a, b, c, d non negative integers (since each of them has at least 2 vouchers). If n is the total number of vouchers, then: a+b+c+d=n8 or a+b+c+d=m with m=n8 Let X=a+b and Y=c+d so X+Y=m and (m+1) possible couples are (X=0, Y=m), (X=1, Y=m1), ..., (X=m, Y=0) On the other hand, (a, b) such that a+b=k and a,b>=0, the number of such couples is equal to k+1. Therefore, generic solution is: 1*(m+1)+2*m+3*(m1)+...+m*2+(m+1)*1 To get 120 different ways, m equals to 7, or n=15, thus C. Sure this is not the easiest way to deal with this problem! Actually there is a formula for this The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items = n+r1Cr1. Here if each nephew gets at least 2 tickets we have n8 tickets left to distribute among 4 nephews and each one of them can receive 0,1,2..n8 items. Total no of ways that can happen = (n8)+(41)C(41) = n5C3 n5C3=120 for n=15 .So C
_________________
___________________________________ Please give me kudos if you like my post




Intern
Joined: 03 Mar 2010
Posts: 34

Re: Voucher!!!
[#permalink]
Show Tags
29 Jul 2010, 15:06
It took me 10 minutes to solve it First, it is remarked that all vouchers are equivalent. Let's call nephews A, B, C, D and their number of vouchers (2+a), (2+b), (2+c), and (2+d), respectively, where a, b, c, d non negative integers (since each of them has at least 2 vouchers). If n is the total number of vouchers, then: a+b+c+d=n8 or a+b+c+d=m with m=n8 Let X=a+b and Y=c+d so X+Y=m and (m+1) possible couples are (X=0, Y=m), (X=1, Y=m1), ..., (X=m, Y=0) On the other hand, (a, b) such that a+b=k and a,b>=0, the number of such couples is equal to k+1. Therefore, generic solution is: 1*(m+1)+2*m+3*(m1)+...+m*2+(m+1)*1 To get 120 different ways, m equals to 7, or n=15, thus C. Sure this is not the easiest way to deal with this problem!
_________________
Hardworkingly, you like my post, so kudos me.



Intern
Joined: 09 Dec 2008
Posts: 22
Location: Vietnam
Schools: Somewhere

Re: Voucher!!!
[#permalink]
Show Tags
29 Jul 2010, 16:04
bogos wrote: It took me 10 minutes to solve it First, it is remarked that all vouchers are equivalent. Let's call nephews A, B, C, D and their number of vouchers (2+a), (2+b), (2+c), and (2+d), respectively, where a, b, c, d non negative integers (since each of them has at least 2 vouchers). If n is the total number of vouchers, then: a+b+c+d=n8 or a+b+c+d=m with m=n8 Let X=a+b and Y=c+d so X+Y=m and (m+1) possible couples are (X=0, Y=m), (X=1, Y=m1), ..., (X=m, Y=0) On the other hand, (a, b) such that a+b=k and a,b>=0, the number of such couples is equal to k+1. Therefore, generic solution is: 1*(m+1)+2*m+3*(m1)+...+m*2+(m+1)*1 To get 120 different ways, m equals to 7, or n=15, thus C. Sure this is not the easiest way to deal with this problem! Anyway, this is a good way to solve this tough problem! Thanks



Intern
Joined: 09 Dec 2008
Posts: 22
Location: Vietnam
Schools: Somewhere

Re: Voucher!!!
[#permalink]
Show Tags
29 Jul 2010, 19:57
Bunuel wrote: dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! Clearly there are more than 8 vouchers as each of four can get at least 2. So, basically 120 ways vouchers can the distributed are the ways to distribute \(x8\) vouchers, so that each can get from zero to \(x8\) as at "least 2", or 2*4=8, we already booked. Let \(x8\) be \(k\). In how many ways we can distribute \(k\) identical things among 4 persons? Well there is a formula for this but it's better to understand the concept. Let \(k=5\). And imagine we want to distribute 5 vouchers among 4 persons and each can get from zero to 5, (no restrictions). Consider: \(ttttt\) We have 5 tickets (t) and 3 separators between them, to indicate who will get the tickets: \(ttttt\) Means that first nephew will get all the tickets, \(ttttt\) Means that first got 0, second 1, third 3, and fourth 1 And so on. How many permutations (arrangements) of these symbols are possible? Total of 8 symbols (5+3=8), out of which 5 \(t\)'s and 3 \(\)'s are identical, so \(\frac{8!}{5!3!}=56\). Basically it's the number of ways we can pick 3 separators out of 5+3=8: \(8C3\). So, # of ways to distribute 5 tickets among 4 people is \((5+41)C(41)=8C3\). For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\). > \(k=7\). Plus the 8 tickets we booked earlier: \(x=k+8=7+8=15\). Answer: C (15). P.S. How to solve \((k+1)(k+2)(k+3)=3!*120=720\): 720, three digit integer ending with 0, is the product of three consecutive integers. Obviously one of them must be multiple of 5: try \(5*6*7=210<720\), next possible triplet \(8*9*10=720\), OK. So \(k+1=8\) > \(k=7\). P.P.S. Direct formula: The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items is \(n+r1C_{r1}\).
The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\).Hope it helps. Thanks for your great explanation!



Manager
Joined: 20 Dec 2010
Posts: 164
Schools: UNC Duke Kellogg

Re: Voucher!!!
[#permalink]
Show Tags
02 Jul 2011, 18:53
5 stars for the "stars and bars" method! The coolest trick ever!



Manager
Joined: 11 Apr 2011
Posts: 72

Re: Voucher!!!
[#permalink]
Show Tags
06 Jul 2011, 11:22
really great trick!!



Intern
Joined: 28 Mar 2011
Posts: 23

Re: Voucher!!!
[#permalink]
Show Tags
23 Jul 2011, 23:52
Hi Bunuel,
Why do we assume k=5?
What would happen if we assume some other value of k?
Thanks!



Intern
Joined: 13 Oct 2012
Posts: 39
Concentration: General Management, Leadership

Re: Voucher!!!
[#permalink]
Show Tags
07 Jan 2013, 12:04
this formula is worth remembering: The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items = n+r1Cr1.
Thanks a lot



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 9803
Location: Pune, India

Re: Voucher!!!
[#permalink]
Show Tags
07 Jan 2013, 21:10
dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! This question is a spin on 'distribute n identical things among m people (some people may get none)' Since she must give each of the 4 nephews atleast 2 vouchers, she must have atleast 8. But she can split the vouchers in 120 ways so she certainly has more than 8. Let's assume she gives 2 to each nephew and has the leftover vouchers in her hand. This becomes your standard 'distribute n identical things among m people (some people may get none)' question. Assume the answer is 15 (it's in the middle) and she has 15  8 = 7 vouchers left in her hand. In how many ways can she distribute them among 4 people? In 10!/7!*3! = 10*9*8/(3*2) = 120 ways. This is the required answer so (C) This concept is explained in detail here: http://www.veritasprep.com/blog/2011/12 ... 93part1/Check method II or question 2 on the link above. Note: Say, you had obtained a number less than 120. You would have assumed 16 next and repeated the calculation. Had you obtained a number more than 120, you would have assumed 13 or 14 and done the calculation. 2 iterations would have certainly given you the answer.
_________________
Karishma Veritas Prep GMAT Instructor
Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options >



Intern
Joined: 07 Sep 2014
Posts: 17
Location: United States (MA)
Concentration: Finance, Economics

Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
20 Oct 2014, 13:01
Bunuel wrote: dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! Clearly there are more than 8 vouchers as each of four can get at least 2. So, basically 120 ways vouchers can the distributed are the ways to distribute \(x8\) vouchers, so that each can get from zero to \(x8\) as at "least 2", or 2*4=8, we already booked. Let \(x8\) be \(k\). In how many ways we can distribute \(k\) identical things among 4 persons? Well there is a formula for this but it's better to understand the concept. Let \(k=5\). And imagine we want to distribute 5 vouchers among 4 persons and each can get from zero to 5, (no restrictions). Consider: \(ttttt\) We have 5 tickets (t) and 3 separators between them, to indicate who will get the tickets: \(ttttt\) Means that first nephew will get all the tickets, \(ttttt\) Means that first got 0, second 1, third 3, and fourth 1 And so on. How many permutations (arrangements) of these symbols are possible? Total of 8 symbols (5+3=8), out of which 5 \(t\)'s and 3 \(\)'s are identical, so \(\frac{8!}{5!3!}=56\). Basically it's the number of ways we can pick 3 separators out of 5+3=8: \(8C3\). So, # of ways to distribute 5 tickets among 4 people is \((5+41)C(41)=8C3\). For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\). > \(k=7\). Plus the 8 tickets we booked earlier: \(x=k+8=7+8=15\). Answer: C (15). P.S. How to solve \((k+1)(k+2)(k+3)=3!*120=720\): 720, three digit integer ending with 0, is the product of three consecutive integers. Obviously one of them must be multiple of 5: try \(5*6*7=210<720\), next possible triplet \(8*9*10=720\), OK. So \(k+1=8\) > \(k=7\). P.P.S. Direct formula: The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items is \(n+r1C_{r1}\).
The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\).Hope it helps. Bunuel, Blast from the past, but I'm just a bit unclear about the rearrangement here, and why/how the k! cancels out when we multiply by the 3! For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\)



Math Expert
Joined: 02 Sep 2009
Posts: 59268

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
20 Oct 2014, 14:10
bsmith37 wrote: Bunuel wrote: dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! Clearly there are more than 8 vouchers as each of four can get at least 2. So, basically 120 ways vouchers can the distributed are the ways to distribute \(x8\) vouchers, so that each can get from zero to \(x8\) as at "least 2", or 2*4=8, we already booked. Let \(x8\) be \(k\). In how many ways we can distribute \(k\) identical things among 4 persons? Well there is a formula for this but it's better to understand the concept. Let \(k=5\). And imagine we want to distribute 5 vouchers among 4 persons and each can get from zero to 5, (no restrictions). Consider: \(ttttt\) We have 5 tickets (t) and 3 separators between them, to indicate who will get the tickets: \(ttttt\) Means that first nephew will get all the tickets, \(ttttt\) Means that first got 0, second 1, third 3, and fourth 1 And so on. How many permutations (arrangements) of these symbols are possible? Total of 8 symbols (5+3=8), out of which 5 \(t\)'s and 3 \(\)'s are identical, so \(\frac{8!}{5!3!}=56\). Basically it's the number of ways we can pick 3 separators out of 5+3=8: \(8C3\). So, # of ways to distribute 5 tickets among 4 people is \((5+41)C(41)=8C3\). For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\). > \(k=7\). Plus the 8 tickets we booked earlier: \(x=k+8=7+8=15\). Answer: C (15). P.S. How to solve \((k+1)(k+2)(k+3)=3!*120=720\): 720, three digit integer ending with 0, is the product of three consecutive integers. Obviously one of them must be multiple of 5: try \(5*6*7=210<720\), next possible triplet \(8*9*10=720\), OK. So \(k+1=8\) > \(k=7\). P.P.S. Direct formula: The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items is \(n+r1C_{r1}\).
The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\).Hope it helps. Bunuel, Blast from the past, but I'm just a bit unclear about the rearrangement here, and why/how the k! cancels out when we multiply by the 3! For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\) (k+3)! = k!*(k+1)(k+2)(k+3), so \(\frac{(k+3)!}{k!3!}=\frac{k!*(k+1)(k+2)(k+3)}{k!3!}=\frac{(k+1)(k+2)(k+3)}{3!}\).
_________________



Intern
Joined: 07 Sep 2014
Posts: 17
Location: United States (MA)
Concentration: Finance, Economics

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
20 Oct 2014, 15:00
Quote: (k+3)! = k!*(k+1)(k+2)(k+3), so \(\frac{(k+3)!}{k!3!}=\frac{k!*(k+1)(k+2)(k+3)}{k!3!}=\frac{(k+1)(k+2)(k+3)}{3!}\).
that is what i figured. thank you.



Director
Joined: 17 Dec 2012
Posts: 623
Location: India

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
26 Oct 2015, 22:51
Let us assume the total number of vouchers is 15. Each nephew should get at least 2 vouchers. The distribution can contain three 2 vouchers, two 2 vouchers , one two voucher or no 2 voucher. For three 2 vouchers the possibilities are 2 2 2 9, 2 2 9 2, 2 9 2 2 and 9 2 2 2 . So there are 4 possibilities. For two 2 vouchers, one 2 voucher and no two voucher, the possibilities can be shown to be 36, 60 and 20. So the total number of possibilities are 120 as required. Even if we did not choose 15 as the number of vouchers we can arrive at it with a couple of choices.
_________________
Srinivasan Vaidyaraman Sravna Test Prep http://www.sravnatestprep.comHolistic and Systematic Approach



Intern
Joined: 27 Sep 2015
Posts: 15

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
26 Dec 2015, 09:45
I was wondering does the question mean 1. each nephew gets 2 vouchers first, 2. the remaining vouchers can be distributed in 120 ways.
so each gets 2 first. 2*4=8 then the remaining vouchers can be distributed in 120 ways (K+3)! / K!*3! = 120 (K+3) (K+2) (K+1) = 720 plug in numbers K = 7 7+ the original 8 = 15. is there anything wrong with my understanding ? thanks



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 9803
Location: Pune, India

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
29 Dec 2015, 02:41
VeritasPrepKarishma wrote: dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! This question is a spin on 'distribute n identical things among m people (some people may get none)' Since she must give each of the 4 nephews atleast 2 vouchers, she must have atleast 8. But she can split the vouchers in 120 ways so she certainly has more than 8. Let's assume she gives 2 to each nephew and has the leftover vouchers in her hand. This becomes your standard 'distribute n identical things among m people (some people may get none)' question. Assume the answer is 15 (it's in the middle) and she has 15  8 = 7 vouchers left in her hand. In how many ways can she distribute them among 4 people? In 10!/7!*3! = 10*9*8/(3*2) = 120 ways. This is the required answer so (C) This concept is explained in detail here: http://www.veritasprep.com/blog/2011/12 ... 93part1/Check method II or question 2 on the link above. Note: Say, you had obtained a number less than 120. You would have assumed 16 next and repeated the calculation. Had you obtained a number more than 120, you would have assumed 13 or 14 and done the calculation. 2 iterations would have certainly given you the answer. Quote: I have one confusion though. So, when we distribute the remaining 7 vouchers we are dividing by 7! and 3!. These take care of the arrangements with 7 identical vouchers and 3 identical bars (vvvvvvv), right?. But what about arrangements like vvvvvvv? Here, how do we take care of the identical arrangements with vvv, vv? If we do not take care of them, then with vvv, there will be additional 3! = 6 arrangements multiplied by 2! arranments for vv, right?
Can you please clarify? I have this confusion for other similar porblems. Maybe I am missing some point here.
Consider this question: How many words can you make using all letters of GOSSIP? This is 6!/2! How about number of words using all letters of MMIISS? This is 6!/2!*2!*2! (this includes all words such as MIISMS, ISMISM, ISSMMI etc) On the same lines, consider VVVVVVVIII  Is it any different from above? This would be 10!/7!*3! (this would include all words such as VVVIVIVVIV) We have already taken care of all identical Vs when we divide by 7!
_________________
Karishma Veritas Prep GMAT Instructor
Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options >



Intern
Joined: 19 Oct 2014
Posts: 36
Location: India
Concentration: Finance, Entrepreneurship
GMAT 1: 600 Q48 V25 GMAT 2: 670 Q49 V31
GPA: 3.26
WE: Operations (Manufacturing)

Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
24 Apr 2018, 01:37
Bunuel wrote: P.P.S. Direct formula:
The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items is \(n+r1C_{r1}\).
The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\).
Hope it helps.
Hi Bunuel, Can you help explain the formula(or share the link if it's already done) for finding the number of ways of dividing n identical items among r persons, when each one of whom receives at least one item ? In this case, how does the concept of separator work. To me, it suggests the formula will have to be \(n+rC_{r}\). Can you help me please understand this.
_________________
"Don't be satisfied with stories, how things have gone with others. Unfold your own myth."Rumi
Consider hitting on kudos button for a sec, if this post helps you.



Intern
Joined: 28 Jul 2018
Posts: 11

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
25 Feb 2019, 04:33
Bunuel wrote: dungtd wrote: Mrs. Smith has been given film vouchers. Each voucher allows the holder to see a film without charge. She decides to distribute them among her four nephews so that each nephew gets at least two vouchers. How many vouchers has Mrs. Smith been given if there are 120 ways that she could distribute the vouchers? (A) 13 (B) 14 (C) 15 (D) 16 (E) more than 16
Could someone help me out? Thanks a lot!!! Clearly there are more than 8 vouchers as each of four can get at least 2. So, basically 120 ways vouchers can the distributed are the ways to distribute \(x8\) vouchers, so that each can get from zero to \(x8\) as at "least 2", or 2*4=8, we already booked. Let \(x8\) be \(k\). In how many ways we can distribute \(k\) identical things among 4 persons? Well there is a formula for this but it's better to understand the concept. Let \(k=5\). And imagine we want to distribute 5 vouchers among 4 persons and each can get from zero to 5, (no restrictions). Consider: \(ttttt\) We have 5 tickets (t) and 3 separators between them, to indicate who will get the tickets: \(ttttt\) Means that first nephew will get all the tickets, \(ttttt\) Means that first got 0, second 1, third 3, and fourth 1 And so on. How many permutations (arrangements) of these symbols are possible? Total of 8 symbols (5+3=8), out of which 5 \(t\)'s and 3 \(\)'s are identical, so \(\frac{8!}{5!3!}=56\). Basically it's the number of ways we can pick 3 separators out of 5+3=8: \(8C3\). So, # of ways to distribute 5 tickets among 4 people is \((5+41)C(41)=8C3\). For \(k\) it will be the same: # of ways to distribute \(k\) tickets among 4 persons (so that each can get from zero to \(k\)) would be \((K+41)C(41)=(k+3)C3=\frac{(k+3)!}{k!3!}=120\). \((k+1)(k+2)(k+3)=3!*120=720\). > \(k=7\). Plus the 8 tickets we booked earlier: \(x=k+8=7+8=15\). Answer: C (15). P.S. How to solve \((k+1)(k+2)(k+3)=3!*120=720\): 720, three digit integer ending with 0, is the product of three consecutive integers. Obviously one of them must be multiple of 5: try \(5*6*7=210<720\), next possible triplet \(8*9*10=720\), OK. So \(k+1=8\) > \(k=7\). P.P.S. Direct formula: The total number of ways of dividing n identical items among r persons, each one of whom, can receive 0,1,2 or more items is \(n+r1C_{r1}\).
The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\).Hope it helps. BunuelHi, let's assume that Mr. Smith has 15 vouchers and 4 nephews whom she want to give the vouchers with the restriction that everyone gets at least 2 vouchers. No we should use the formula ("The total number of ways of dividing n identical items among r persons, each one of whom receives at least one item is \(n1C_{r1}\)" This would be \(152C_{41}\). This does not result in 120. Where am I wrong?



Manager
Joined: 23 Aug 2017
Posts: 120

Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
Show Tags
25 Feb 2019, 07:35
Bunuel VeritasKarishmaWhy dont we consider the no of ways in which 8 vouchers are to be distributed among the 4 nephews equally? Isnt that also included within 120 ways? Please explain




Re: Mrs. Smith has been given film vouchers. Each voucher allows
[#permalink]
25 Feb 2019, 07:35



Go to page
1 2
Next
[ 28 posts ]



