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Ninety-students-including-Joe-and-Jane-are-to-be-split-into-three-classes-of-equal-size-and-is-to-be-done-at-random-What-is-the-probability-that-Joe-and-Jane-end-up-in-the-same-class-




Question Number 131975 by liberty last updated on 10/Feb/21
 Ninety students , including Joe  and Jane are to be split into   three classes of equal size and is  to be done at random. What is  the probability that Joe and Jane  end up in the same class ?
$$\:\mathrm{Ninety}\:\mathrm{students}\:,\:\mathrm{including}\:\mathrm{Joe} \\ $$$$\mathrm{and}\:\mathrm{Jane}\:\mathrm{are}\:\mathrm{to}\:\mathrm{be}\:\mathrm{split}\:\mathrm{into}\: \\ $$$$\mathrm{three}\:\mathrm{classes}\:\mathrm{of}\:\mathrm{equal}\:\mathrm{size}\:\mathrm{and}\:\mathrm{is} \\ $$$$\mathrm{to}\:\mathrm{be}\:\mathrm{done}\:\mathrm{at}\:\mathrm{random}.\:\mathrm{What}\:\mathrm{is} \\ $$$$\mathrm{the}\:\mathrm{probability}\:\mathrm{that}\:\mathrm{Joe}\:\mathrm{and}\:\mathrm{Jane} \\ $$$$\mathrm{end}\:\mathrm{up}\:\mathrm{in}\:\mathrm{the}\:\mathrm{same}\:\mathrm{class}\:? \\ $$
Commented by liberty last updated on 10/Feb/21
i got n(S)= ((90!)/(30!.30!.30!))  n(A)=3×C_(28) ^(88) ×C_(30) ^(60) ×C_(30) ^(30) =3×((88!)/(28!.60!))×((60!)/(30!.30!))  P(A)= ((3×((88!)/(28!.))×30!)/(90!))=((3×30×29)/(90×89))      = ((29)/(89))
$$\mathrm{i}\:\mathrm{got}\:\mathrm{n}\left(\mathrm{S}\right)=\:\frac{\mathrm{90}!}{\mathrm{30}!.\mathrm{30}!.\mathrm{30}!} \\ $$$$\mathrm{n}\left(\mathrm{A}\right)=\mathrm{3}×\mathrm{C}_{\mathrm{28}} ^{\mathrm{88}} ×\mathrm{C}_{\mathrm{30}} ^{\mathrm{60}} ×\mathrm{C}_{\mathrm{30}} ^{\mathrm{30}} =\mathrm{3}×\frac{\mathrm{88}!}{\mathrm{28}!.\mathrm{60}!}×\frac{\mathrm{60}!}{\mathrm{30}!.\mathrm{30}!} \\ $$$$\mathrm{P}\left(\mathrm{A}\right)=\:\frac{\mathrm{3}×\frac{\mathrm{88}!}{\mathrm{28}!.}×\mathrm{30}!}{\mathrm{90}!}=\frac{\mathrm{3}×\mathrm{30}×\mathrm{29}}{\mathrm{90}×\mathrm{89}} \\ $$$$\:\:\:\:=\:\frac{\mathrm{29}}{\mathrm{89}} \\ $$$$ \\ $$
Commented by liberty last updated on 10/Feb/21
hahaha...i′m misread the question
$$\mathrm{hahaha}…\mathrm{i}'\mathrm{m}\:\mathrm{misread}\:\mathrm{the}\:\mathrm{question} \\ $$
Answered by TheSupreme last updated on 10/Feb/21
Joe is in the class x  P(Jane∈x)=((30−1)/(90−1))=((29)/(89))
$${Joe}\:{is}\:{in}\:{the}\:{class}\:{x} \\ $$$${P}\left({Jane}\in{x}\right)=\frac{\mathrm{30}−\mathrm{1}}{\mathrm{90}−\mathrm{1}}=\frac{\mathrm{29}}{\mathrm{89}} \\ $$$$ \\ $$
Commented by liberty last updated on 10/Feb/21
haha its general formula
$$\mathrm{haha}\:\mathrm{its}\:\mathrm{general}\:\mathrm{formula} \\ $$

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