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A chess team of 2 girls and 2 boys is to be chosen from the 7 girls and 6 boys in the...

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jspake | Student, Undergraduate | eNoter

Posted April 24, 2013 at 2:41 PM via web

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A chess team of 2 girls and 2 boys is to be chosen from the 7 girls and 6 boys in the chess club.
Find the number of ways this can be done if 2 of the girls are twins and are either both in the team or both not in the team.

Please help me solve the question above.. an explanation will be appreciated.
Thanks

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pramodpandey | College Teacher | Valedictorian

Posted April 24, 2013 at 3:59 PM (Answer #1)

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No of Boys=  6    ,No of Girls =7

Team  2 boys  and 2 girls

Cases

(i) Twins are in team

(ii) Twins are not in team

I . Twins are in team

 Girls can be selected C(2,2) 

 Boys can be selected C(6,2)

By Fundamental Principal of counting  ,Total no. of possible selection

= C(2,2) x C(6,2)                  (i)

II Twins are not in Team

Girls can be selected = C(5,2)

Boys can be selected = C(6,2)

By Fundamental Principal of counting  ,Total no. of possible selection

= C(5,2) x C(6,2)                  (ii)

Thus total no. of possible selection of team= C(2,2)xC(6,2)+C(5,2)xC(6,2)

=C(6,2) {C(2,2)+C(5,2)}

=15 x(1+10)

=15 x11

=165

 

 

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tiburtius | High School Teacher | (Level 3) Associate Educator

Posted April 24, 2013 at 4:01 PM (Answer #2)

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Since twins are either chosen both or none we look at them separately. From the rest of the 5 girls we can choose 2 in 

`((5),(2))=(5cdot4)/(1cdot2)=10` ways and from twins we can choose in only 1 way. So we can choose 2 girls in 10+1=11 ways.

Similarly we can choose 2 boys from 6 in `((6),(2))=(6cdot5)/(1cdot2)=15` ways.

So we can choose 2 girls and 2 boys in `11cdot15=165` ways.

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