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rishabh2310

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sahilsah456
­The set {s, t, u} has the subsets {s}, {t}, {u}, {s,t}, {s,u}, {t,u}, {s,t,u} and {} - empty set. What is the number of the subsets of set {s,t,u,v,w} that must be exclude element t and include element u?
A. 2
B. 4
C. 8
D. 16
E. 32
­A straight formula for such questions is \(2^{number \ \ of \ \ element \ \ in \ \ the \ \ set}\).

The number of the subsets of set {s,t,u,v,w} that must be exclude element t and include element u: So exclude both t and u. We are left with {s, v, w} and total number of ways = \(2^3\) or 8.

Let us find the solution by calculating.

We cannot include t, so we are left with {s,u,v,w}.
Next, u is always there, so {u, X}. This X would be combination of the remaining.
1) Only one element: {u}, here X is null.
2) Only two element: {u, X}, choose X as any one of the remaining elements, so 3C1 or 3 ways. => {u, s}, {u, v}, {u, w}
3) Only three element: {u, X}, choose X as any two of the remaining elements, so 3C2 or 3 ways. => {u, s, v}, {u, v, w}, {u, s, w}
4) Only four element: {u, X}, choose X as any three of the remaining element, so 3C3 or 1 way. => {u, s, v, w}

Total = 1+3+3+1 = 8

C
­why did u exlcude u from the 2^3 formule the questions asks u to include u and exclude t so why do we exclude u while calculating using the formulae ???? please explain
Because u is not affecting the answer. It is always there­, so there is only one option for it, and that is YES, while for other three, the options are Yes and No, so TWO of them.
Answer: 1*2*2*2
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What is the number of the subsets of set {s,t,u,v,w} that must exclude element t and include element u?

1. Set of One Element: u = 1
2. Set of Two Elements: one Must be u and other must be from (s,v,w) = 3C1 * 1 = 3
3. Set of Three Elements: one Must be u and other two must be from (s,v,w) = 3C2 * 1 = 3
4. Set of Four Elements: one Must be u and other Three must be from (s,v,w) = 3C3 * 1 = 1

Total Cases: 1+3+3+1 = 8.

Ans. C
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