Find the number of ways in which 12 children can be divided into 2 groups of 6 if two particular boys must be in different groups.

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william1941's profile pic

william1941 | College Teacher | (Level 3) Valedictorian

Posted on

The number of ways of choosing 6 boys from the 12 to form the two groups of 6 boys is given by C(12,6)

In the case where the two boys are in one group, the number of ways of forming the groups is C(10, 4). This is due to the fact that 2 boys are taken together for a group, so now we need to choose another 4 boys from the 10 remaining to complete the group.

So the number of ways of forming the groups where the two boys are in different groups can be calculated by subtracting the case where they are together from the general case: this is given by C(12, 6) - C(10 , 6)

= 12! / (6!* 6!) - 10! / (6!*4!)

= 924 - 210

= 714

Therefore the number of ways the children can be divided is 714.

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neela | High School Teacher | (Level 3) Valedictorian

Posted on

The number of ways of forming 6 boys 0f two groups in which two patcular boys must be in dofferent groups.

We know that when we are doing 6 boys of two groups it sufficient if we do the combination 6 of one group. The remaining unselected 6 boys is automatcally another group of six.

Since we want two particular boys in different groups of six, we identify them  and keep them separate. From the  rest of the 10 boys we can  select 5 . After selecting 5  we add one of the  separately kept 2 boys to the the selected group.  The other boy automatically will be  among  the unseled boys. Thus we do this activity in 10C5  Or C(10 , 5)* 2 ways.

So the number of possible ways of grouping  of 6 out of 12 with 2 particualar boys  in separate groups is 10C5 = (10*9*8*7*6/5!)*2 = 504.

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