Team of four students is to be selected from a total of 12 students for a quiz competition. In how many ways it can be selected if two particular students refuse to be together and two particular students wish to be together only in the team. .
226
We suppose that student A and B wish to be selected together and students C and D don't wish to be together.
Case 1: Select all four students from remaining 8 students
Number of ways = = 70
Case 2: A and B are selected and C and D are not selected.
Select reaming 2 from reaming 8.
Number of ways = = 28
Case 3: A, B and C are selected D is not selected.
Select 1 from reaming 8.
Number of ways = = 8
Case 4: A, B and D are selected C is not selected.
Select 1 from reaming 8.
Number of ways = = 8
Case 5: D is selected and A, B and C are not selected.
Select 3 from reaming 8.
Number of ways = = 56
Case 6: C is selected and A, B and D are not selected.
Select 3 from reaming 8.
Number of ways = = 56
Total number of ways =case 1 + case 2 +case 3 +case 4 + case 5 + case 6
=70+28+8+8+56+56=226