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Sub sets probability

This topic has 5 expert replies and 4 member replies

Sub sets probability

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Hello,

Can you please assist with this:

The subsets of set {s,u,t} are {s},{u},{t},{su},{st},{ut},{s,u,t},and the
empty set { }. How many of the subsets of set {s,u,t,w, x}do NOT contain element t?

(A) 13
(B) 14
(C) 15
(D) 15
(E) 16

OA: E

Thanks a lot,
Sri

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gmattesttaker2 wrote:
Hello,

Can you please assist with this:

The subsets of set {s,u,t} are {s},{u},{t},{su},{st},{ut},{s,u,t},and the
empty set { }. How many of the subsets of set {s,u,t,w, x}do NOT contain element t?

(A) 13
(B) 14
(C) 15
(D) 15
(E) 16
This is very similar to this question: http://www.beatthegmat.com/combinatorics-problem-t273180.html


Here's one approach:

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 = E)

Cheers,
Brent

Aside: For more information about the FCP, watch our free video: http://www.gmatprepnow.com/module/gmat-counting?id=775

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first approach
no of subsets in any set are 2^n where n is the number of elements in given set
just remove t from superset and now new set have 4 elements therefore no of subsets = 2^4 = 16

second approach
just remove t from superset and now new set have 4 elements therefore no of subsets
with one element 4c1 + with 2 4c2+ with 3 4c3 + with 4 4c4 + 1empty set = 16

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vipulgoyal wrote:
first approach
no of subsets in any set are 2^n where n is the number of elements in given set
just remove t from superset and now new set have 4 elements therefore no of subsets = 2^4 = 16

second approach
just remove t from superset and now new set have 4 elements therefore no of subsets
with one element 4c1 + with 2 4c2+ with 3 4c3 + with 4 4c4 + 1empty set = 16
vipulgoyal's solution demonstrates a useful property regarding sums of combinations:
nC0 + nC1 + nC2 + . . . . nCn = 2^n

So, for this question, we have: 4C0 + 4C1 + 4C2 + 4C3 + 4C4 = 2^4 = 16
Likewise, 5C0 + 5C1 + 5C2 + 5C3 + 5C4 + 5C5 = 2^5
And 7C0 + 7C1 + 7C2 + . . . 7C6 + 7C7 = 2^7
etc.

Cheers,
Brent

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why not the answer can be 13??
as we have 7 subsets of {s,u,t} and if we list all the subsets of {s,u,t,w,x} the total number of subsets are 16. by excluding the subsets which contain t, we get the answer 13.

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konan wrote:
why not the answer can be 13??
as we have 7 subsets of {s,u,t} and if we list all the subsets of {s,u,t,w,x} the total number of subsets are 16. by excluding the subsets which contain t, we get the answer 13.
Here's the list.

{} {s} {u} {w} {x} {su} {sw} {sx} {uw} {ux} {wx} {suw} {sux} {swx} {uwx} {suwx}

Which ones did you miss? I didn't think of {suwx} at first myself.

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konan wrote:
why not the answer can be 13??
as we have 7 subsets of {s,u,t} and if we list all the subsets of {s,u,t,w,x} the total number of subsets are 16. by excluding the subsets which contain t, we get the answer 13.
There are 32 subsets of {s, u, t, w, x}: for each subset, we have two choices (s is in or s in out, t is in or t is out, etc.), which gives us 2⁵ or 32 subsets. (If you don't want to consider {s, u, t, w, x} itself, you could say there are 31 proper subsets, but 32 is still the correct answer.)

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Using combination i.e. "selection" logic.

No. of ways to select Subset that doesn't contain t = (All subset) - (Subset always contains t) ---(1)

All Subset = 5C5 + 5C4 + 5C3 + 5C2 + 5C1 = 1 + 5 + 10 + 10 + 5 = 31

Subset always contains t = 4C4 +4C3 + 4C2 + 4C1 = 1 + 4 + 6 + 4 = 15

From (1), Answer is 16.

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You can also use the same logic Matt uses for the full set. If you exclude t, you'd have 4 elements in the set. You have 2 options for each element: in or out of the set. 2^4 = 16.

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Just realized there's a blunderous typo in my post:

Quote:
There are 32 subsets of {s, u, t, w, x}: for each subset, we have two choices
is technically correct, I suppose, but not in the sense I intended. I meant to say

Quote:
There are 32 subsets of {s, u, t, w, x}: for each element in the set, we have two choices
My bad if that confused anybody, I know this is hard enough to follow as is!

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