SOLUTION: Lili has 20 friends. Among them are Kevin and Gerry, who are husband and wife. Lili wants to invite six of her friends to her birthday party. If neither Kevin nor Gerry will go t

Question 1122854: Lili has 20 friends. Among them are Kevin and Gerry, who are husband and wife.
Lili wants to invite six of her friends to her birthday party. If neither Kevin nor
Gerry will go to a party without the other, how many choices does Lili have?
Answer by ikleyn(52810) (Show Source): You can put this solution on YOUR website!
.

If each separate "choice" is the group of 6 invited friends, then the solution is THIS :

    the number of choices Lili has =  + 


    where first addend in the formula represents  "6 of 18 including NEITHER Kevin NOR Gerry", 

    while the second addend in the formula represents "4 of 18 assuming that both Kevin and Gerry are included".

I assume that you are familiar with Combinations to that extent to calculate the numbers from the formula on your own.

If not - then you have a good chance and a happy opportunity to learn about Combinations from introductory lessons
    - Introduction to Combinations
    - PROOF of the formula on the number of Combinations
    - Problems on Combinations
in this site.

Also, you have this free of charge online textbook in ALGEBRA-II in this site
    - ALGEBRA-II - YOUR ONLINE TEXTBOOK.

The referred lessons are the part of this online textbook under the topic "Combinatorics: Combinations and permutations".

Save the link to this textbook together with its description

Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson

into your archive and use when it is needed.

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