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If you are given these options for dinner:

  • Pie
  • Cake
  • Muffin
  • Ice Cream

And you can have any of these options as you like and you can't have more than one of the same item. Then what are all the possible combinations?

For example you can have:

Pie
Pie, Cake
Pie, Cake, Muffin
Pie, Cake, Muffin, Ice Cream
Pie, Ice Cream
Cake, Muffin

etc...

Is there a way to find all possible combinations?

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  • $\begingroup$ Do you want to list all of the combinations, or do you simply want to know how many there are? $\endgroup$ Commented May 5, 2012 at 9:11
  • $\begingroup$ @Brian M. Scott I would like to know the list of all possible combinations not just the number. $\endgroup$
    – Sam
    Commented May 5, 2012 at 9:13
  • $\begingroup$ Yes, it is even possible to write a computer program that does it, not the most efficient though ... $\endgroup$
    – rtybase
    Commented Jun 11, 2019 at 17:27
  • $\begingroup$ The answers below are good but I always fall back on binary, say XXXX then replacing the X's with {0,1} both enumerates and counts. Then label the positions. $\endgroup$
    – rrogers
    Commented Oct 1, 2019 at 21:59
  • $\begingroup$ @Sam - This question is so old, but there are two answers to this question. See my answer below. Just giving the answer of 15 (or 16 with the missing dessert) is only one possible answer. $\endgroup$ Commented Mar 22, 2023 at 9:24

4 Answers 4

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A set of $4$ elements has $2^4$ subsets. These include the empty set, in which case you eat nothing. So the answer is $16$ if eating nothing is an option, or $15$ if eating nothing is not an option. The wording of the problem does not make it clear whether you can choose the empty set of foods.

The numbers are small enough that you could actually enumerate all the possibilities. You were well under way. To be sure of not missing any, you might proceed systematically. For brevity call the foods A, B, C, D.

We can have

i) nothing;

ii) A or B or C or D;

iii) AB or AC or AD or BC or BD or CD;

iv) BCD or ACD or ABD or ABC;

v) ABCD.

Count: we get $16$.

To see the "formula" $2^4$, imagine you are in a cafeteria line and the A's, B's, C's, and D's are lined up in that order. You look at the A's and say yes or no. Then you do the same with the B's, the C's, the D's. The number of possible choices is the number of strings of length $4$ made up of the letters Y (for Yes) and/or N (for No). There are $2^4$ such strings. The option NNNN corresponds to going hungry.

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  • $\begingroup$ Eating nothing is not an option. Could you please provide a list of the 15 possible combinations? $\endgroup$
    – Sam
    Commented May 5, 2012 at 9:15
  • $\begingroup$ I had already edited to provide a list. If the number of options is say $10$ instead of $4$, then counting would still be easy ($2^{10}$, or $2^{10}-1$ if you do not allow the empty cafeteria tray). But making a complete list would be unpleasant, since $2^{10}=1024$. $\endgroup$ Commented May 5, 2012 at 9:23
  • $\begingroup$ @Sam - This question is so old, but there are two answers to this question. See my answer below. Just giving the answer of 15 (or 16 with the missing dessert) is only one possible answer. $\endgroup$ Commented Mar 22, 2023 at 9:23
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Preliminary

  • The subsets of a set $A=A$'s power set $=\mathcal{P}\left(A\right)$.
  • The size of a set $A=A$'s cardinality $=card\left(A\right)$.
  • Let your options be represented by the set $X = \left\{P,C,M,I\right\}$.

This table describes what to do for any basic combinatorics/permutations q...

$$ \begin{array}{l} \begin{array}{c|c} \hskip36.5pt & \hskip42.5pt\style{font-family:inherit}{\text{Ordering}} \end{array} \\[-7pt]\hline\hskip-5.5pt \begin{array}{c|c|c} \style{font-family:inherit}{\text{Repetition}} & \style{font-family:inherit}{\text{w/}} & \style{font-family:inherit}{\text{w/o}} \\\hline \style{font-family:inherit}{\text{w/}} & P_r^n=n^r & C_r^n=\left(\!\left(\begin{smallmatrix} n \\ r \end{smallmatrix}\right)\!\right)=\left(\begin{smallmatrix} n+r-1 \\ r \end{smallmatrix}\right) \\[0pt]\hline \style{font-family:inherit}{\text{w/o}} & nPr=\frac{n!}{(n-r)!} & nCr=\left(\begin{smallmatrix} n \\ r \end{smallmatrix}\right)=\frac{n!}{r!(n-r)!} \end{array}\hskip-5.5pt \end{array} $$

Remark

I will provide my own answer; but first, I will assign some notation to the already accepted answer (which correctly stated that the q could be thought of as the # sequences "(a,b,c,d)" s.t. a,b,c,d $=$ Y/N, i.e., w/ repetition & w/ ordering, subtracted by 1)...

  • $\mathcal{P}\left(X\right) = \left\{\left\{\right\},\left\{P\right\},\left\{C\right\},\left\{M\right\},\left\{I\right\},\left\{P,C\right\},\left\{P,M\right\},\left\{P,I\right\},\left\{C,M\right\},\left\{C,I\right\},\left\{I,M\right\}\left\{P,C,M\right\},\left\{P,C,I\right\},\left\{P,M,I\right\},\left\{C,M,I\right\},\left\{P,C,M,I\right\}\right\}\hskip-2pt\style{font-family:inherit}{\text{.}}$
  • # Combinations w/ empty set $= card\left(\mathcal{P}\left(X\right)\right) = 16$.
  • # Combinations w/o empty set = $card\left(\mathcal{P}\left(X\right)\right) - 1 = 15$.

Answer

Let's consider the q in its intuitive (some would say "raw") form, i.e., 1 about finding the # combinations of options from $X$ w/o repetition or ordering. The table says to use the Binomial Coefficient ($nCr$) but we will have to use it 4 times & + up the results to account for your 4 differently sized subsets...

$$\style{font-family:inherit}{\text{# Combinations}} = 4C1 + 4C2 + 4C3 + 4C4 = \left(\begin{smallmatrix} 4 \\ 1 \end{smallmatrix}\right) + \left(\begin{smallmatrix} 4 \\ 2 \end{smallmatrix}\right) + \left(\begin{smallmatrix} 4 \\ 3 \end{smallmatrix}\right) + \left(\begin{smallmatrix} 4 \\ 4 \end{smallmatrix}\right) = 15$$

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Here you can input your options and get all possible combinations: mathisfun.com

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  • $\begingroup$ This applet lists the $k$-element subsets of a set. So, in order to get all subsets, one would need to run the program for $k=1,2,\ldots,n$. $\endgroup$ Commented Oct 15, 2012 at 12:24
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Assumptions and Question: And you can have any of these options as you like and you can't have more than one of the same item. Then what are all the possible combinations?

Final Answer(s):

All four possible scenarios are below with their formulas. Since one requirement was that we "can't have more than one of the same item", then the two scenarios (of four possible below) where "items cannot repeat" is a valid answer. You do not have a requirement for having order matter.

So the final answer is either 15 or 64. Both are correct. Some people's stomachs don't agree with ice cream in the wrong order, and might cause acid reflux. :)

Key:

A = Pie
B = Cake
C = Muffin
D = Ice Cream

Unique combinations (order does not matter; items cannot repeat):

unique combinations with 1 item
4
unique combinations with 2 items
6
unique combinations with 3 items
4
unique combinations with 4 items
1

Formula:

Reference: https://en.wikipedia.org/wiki/Permutation

n! / ((n-k)!k!)

4! / ((4-1)!1!)
4! / (3!1!)
24 / 6
4

4! / ((4-2)!2!)
4! / (2!2!)
24 / 4
6

4! / ((4-3)!3!)
4! / (1!3!)
24 / 6
4

4! / ((4-4)!4!)
4! / (0!4!)
4! / 4!
1

Here is the enumeration:

1 item in list = 4
A, B, C, D
(1A, 1B, 1C, 1D)

2 items in list = 6
AB, AC, AD, BC, BD, CD
(3A, 2B, 1C, 0D)

3 items in list = 4
(3A, 1B)
ABC, ABD, ACD, BCD

4 items in list = 1
(4A, 0B, 0C, 0D)
ABCD

ANSWER:

15 unique combinations (order does not matter; items cannot repeat)

Unique combinations (order does matter; items cannot repeat):

unique combinations with 1 item
4
unique combinations with 2 items
4 * 3 = 12
unique combinations with 3 items
4 * 3 * 2 = 24
unique combinations with 4 items
4 * 3 * 2 * 1 = 24

Formula:

n! / (n-k)!

4! / (4-1)!
4! / 3!
24 / 6
4

4! / (4-2)!
4! / 2!
24 / 2
12

4! / (4-3)!
4! / 1!
24 / 1
24

4! / (4-4)!
4! / 0!
24

Here is the enumeration:

1 item in list = 4
A, B, C, D
1A, 1B, 1C, 1D

2 items in list = 12 
AB, BA, AC, CA, AD, DA, BC, CB, BD, DB, CD, DC
3A, 3B, 3C, 3D

3 items in list = 24
DAB, DBA, DAC, DCA, CAD, CDA, ABC, ACB, ABD, ADB, ACD, ADC, BAD, CBA, BCA, BDA, CAB, BCD, BAC, BDC, CBD, CDB, DBA, DBC
6A, 6B, 6C, 6D

4 items in list = 24
CDAB, CDBA, BDAC, BDCA, BCAD, BCDA, DABC, DACB, CABD, CADB, BACD, BADC, CBAD, DCBA, DBCA, CBDA, DCAB, ABCD, DBAC, ABDC, ACBD, ACDB, CDBA, ADBC
6A, 6B, 6C, 6D

ANSWER:

64 unique combinations (order does matter; items cannot repeat)

Unique combinations (order does matter; items repeat)

combinations with 1 item
4
combinations with 2 items
4^2 = 4*4 = 16
combinations with 3 items
4^3 = 4*4*4 = 64
combinations with 4 items
4^4 = 4*4*4*4 = 256

ANSWER:

340 unique combinations (order does matter; items repeat)

Unique combinations (order does not matter; items repeat):

combinations with 1 item
4
combinations with 2 items
10
combinations with 3 items
20
combinations with 4 items
35

Formula:

(k + n - 1)! / k!(n - 1)!

(1 + 4 - 1)! / 1!(4 - 1)!
4! / 1!(3!)
24 / 6
4

(2 + 4 - 1)! / 2!(4 - 1)!
5! / 2!3!
120 / 12
10

(3 + 4 - 1)! / 3!(4 - 1)!
6! / 3!3!
6! / 36
20

(4 + 4 - 1)! / 4!(4 - 1)!
7! / 4!3!
5040 / 24*6
35

Enumerations:

1 item in list = 4
A, B, C, D

2 items in list = 10
AA, AB, AC, AD, BB, BC, BD, CC, CD, DD

3 items in list = 20
AAA, AAB, AAC, AAD, ABB, ABC, ABD, ACC, ACD, ADD, BBB, BBC, BBD, BCC, BCD, BDD, CCC, CCD, CDD, DDD

4 items in list = 35
AAAA, AAAB, AAAC, AAAD, AABB, AABC, AABD, AACC, AACD, AADD, ABBB, ABBC, ABBD, ABCC, ABCD, ABDD, ACCC, ACCD, ACDD, ADDD, BBBB, BBBC, BBBD, BBCC, BBCD, BBDD, BCCC, BCCD, BCDD, BDDD, CCCC, CCCD, CCDD, CDDD, DDDD


ANSWER: 

> 69 unique combinations (order does not matter; items repeat)
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