A person has 5 tickets for a concert and she wants to invite 4 of her 8 best friends. How many choices does she have, if two of her friends do not get along and cannot be both invited?
1 answer:
Answer:
55
Step-by-step explanation:
Combinations formula is used to make choice of 'R' out of 'N' options =
N(C)R = N ! / [ R ! . (N-R)! ]
Total choices to choose 4 out of 8 friends = 8C4
= 8! / (4! 4!)
= 70
Choices for calling them 2 together = 2C2 x 6C2
= 1 x [ 6! / (2! 4!)]
= 15
So : Number of choices that the 2 friends are not called together = Total choices - choices they are called together
= 70 - 15 = 55
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