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The subsets of the set {s, t, u} consisting of the three elements s, t

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The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16

Originally posted by saxenarahul021 on 26 Nov 2012, 23:54.
Last edited by Bunuel on 09 Jan 2019, 02:51, edited 2 times in total.
Renamed the topic and edited the tags.
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 27 Nov 2012, 02:02
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


Consider the set without t: {s, u, w, x}. Each subset of this set will be subset of the original set but without t.

# of subsets of {s, u, w, x} is 2^4=16 (each out of 4 element of the set {s, u, w, x} has TWO options: either to be included in the subset or not, so total # of subsets is 2^4=16).

Answer: E.
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 29 Nov 2012, 00:52
Bunuel wrote:
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


Consider the set without t: {s, u, w, x}. Each subset of this set will be subset of the original set but without t.

# of subsets of {s, u, w, x} is 2^4=16 (each out of 4 element of the set {s, u, w, x} has TWO options: either to be included in the subset or not, so total # of subsets is 2^4=16).

Answer: E.


I am a little bit confused by your solution can you please clarify.
I agree we need to find out how many subsets are possible without t, so 4 letter could have 1 set ({SUWX} 4!/4!=1), 3 letters could have 4 sets (4!/3!=4), 2 letters could have 6 sets (4!/2!x2!=6), and 1 letter could have 4 sets. So overall 15 sets and the answer is D. Where did i go wrong?
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 29 Nov 2012, 01:59
ziko wrote:
Bunuel wrote:
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


Consider the set without t: {s, u, w, x}. Each subset of this set will be subset of the original set but without t.

# of subsets of {s, u, w, x} is 2^4=16 (each out of 4 element of the set {s, u, w, x} has TWO options: either to be included in the subset or not, so total # of subsets is 2^4=16).

Answer: E.


I am a little bit confused by your solution can you please clarify.
I agree we need to find out how many subsets are possible without t, so 4 letter could have 1 set ({SUWX} 4!/4!=1), 3 letters could have 4 sets (4!/3!=4), 2 letters could have 6 sets (4!/2!x2!=6), and 1 letter could have 4 sets. So overall 15 sets and the answer is D. Where did i go wrong?


You are forgetting an empty set, which is also a subset of {s, u, w, x} and do not contain t.
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 29 Nov 2012, 04:58
I am a little bit confused by your solution can you please clarify.
I agree we need to find out how many subsets are possible without t, so 4 letter could have 1 set ({SUWX} 4!/4!=1), 3 letters could have 4 sets (4!/3!=4), 2 letters could have 6 sets (4!/2!x2!=6), and 1 letter could have 4 sets. So overall 15 sets and the answer is D. Where did i go wrong?[/quote]

You are forgetting an empty set, which is also a subset of {s, u, w, x} and do not contain t.[/quote]


Thanks Bunuel, i got it, but do you think the way of thinking was correct?
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 29 Nov 2012, 07:27
ziko wrote:
Thanks Bunuel, i got it, but do you think the way of thinking was correct?


Yes, your approach is correct:
# of subsets with 4 elements is 1: \(C^4_4=1\);
# of subsets with 3 elements is 4: \(C^3_4=4\);
# of subsets with 2 elements is 6: \(C^2_4=6\);
# of subsets with 1 elements is 4: \(C^1_4=4\);
plus 1 empty set.

1+4+6+4+1=16.
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 12 May 2014, 13:22
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


I thought that the formula for number of subsets for n elements was 2^n -1

Can anybody explain why this formula does not apply in this case?

Thanks!
Cheers
J :)
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 13 May 2014, 00:03
jlgdr wrote:
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


I thought that the formula for number of subsets for n elements was 2^n -1

Can anybody explain why this formula does not apply in this case?

Thanks!
Cheers
J :)


The number of subsets of a set with n elements is 2^n, including an empty set.
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 06 Dec 2018, 16:16
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saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16

Take the task of building subsets and break it into stages.

Stage 1: Determine whether or not to place "t" in the subset
The question tells us that "t" cannot be in the subset.
So, we can complete stage 1 in 1 way (that is, we DO NOT place "t" in the subset

Stage 2: Determine whether or not to place "s" in the subset
We can either HAVE "s" in the subset or NOT HAVE "s" in the subset
So, we can complete stage 2 in 2 ways

Stage 3: Determine whether or not to place "u" in the subset
We can either HAVE "u" in the subset or NOT HAVE "u" in the subset
So, we can complete this stage in 2 ways

Stage 4: Determine whether or not to place "w" in the subset
We can either HAVE "w" in the subset or NOT HAVE "w" in the subset
So, we can complete this stage in 2 ways

Stage 5: Determine whether or not to place "x" in the subset
We can complete this stage in 2 ways

By the Fundamental Counting Principle (FCP), we can complete the 5 stages (and thus build all possible subsets) in (1)(2)(2)(2)(2) ways (= 16 ways)

Answer: E

Note: the FCP can be used to solve the MAJORITY of counting questions on the GMAT. So, be sure to learn it.

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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 09 Jan 2019, 02:48
1
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


Dear Moderator,
Can we please update the OA for this question, Thank you.
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Re: The subsets of the set {s, t, u} consisting of the three elements s, t  [#permalink]

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New post 09 Jan 2019, 02:52
stne wrote:
saxenarahul021 wrote:
The subsets of the set {s, t, u} consisting of the three elements s, t, and u are {s}, {t}, {u}, {s, t}, {s, u}, {t, u}, {s, t, u}, and the empty set { }. How many different subsets of the set {s, t, u, w, x} do not contain t as an element?

A. 4
B. 7
C. 8
D. 15
E. 16


Dear Moderator,
Can we please update the OA for this question, Thank you.

________________
Done. Thank you.
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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

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Re: The subsets of the set {s, t, u} consisting of the three elements s, t &nbs [#permalink] 09 Jan 2019, 02:52
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