A group of 9 students, s1,s2, ,s9, is to be divided to form three teams X,Y, and Z of sizes 2,3, and 4, respectively. Suppose that s1 cannot be selected for the team X, and s2 cannot be selected for the team Y. Then the number of ways to form such teams, is .
Answer
665
Solution
Use inclusion–exclusion.
Total = 9!2! 3! 4!=1260
s1 X: 81 73=280
s2 Y: 82 62=28 15=420
Both: 71 62=105
Required =1260-280-420+105=665
Answer: 665