I run a book club with $n$ people, not including myself. Every day, for $365$ days, I invite five members in the club to review a book. What is the smallest positive integer $n$ so that I can avoid ever having the exact same group of five members over all $365$ days?

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Answer:

Step-by-step explanation:

if we have n people and we want to do groups of 5, the total number of different combinations is:

[tex]c = \frac{n!}{(n - 5)!5!}[/tex]

find the smallest n such c > 365.

let's do it by brute force:

if n = 5, we have:

[tex]c = 5C_5 = 5[/tex]

if n = 6

[tex]c =6C_5 =6[/tex]

if n = 7

[tex]c = 7C_ 5= 21[/tex]

if n = 8

[tex]c = 8C_5 = 56[/tex]

if n = 9

[tex]C = 9!/(4!*5!) = 126[/tex]

so 9 is not enough, let's see N = 10

C = 10!/(5!*5!)  = 252

10 is not enough, let's see with 11.

C = 11!/(6!*5!) =  462

So you need at least 11 members in the club.

Then the minimum number of members such we have more than 365 combinations is 11 members

Client need al least 11 members in the club.

Permutation and Combination

According to the question,

Group of five members, the combination will be:

→ [tex]c = \frac{n!}{(n-5)! \ 5!}[/tex]

By using Brute force,

If n = 5,

c = 5,

C₅ = 5

If n = 6,

c = 6,

C₅ = 6

If n = 7,

c = 7,

C₅ = 21

If n = 8,

c = 8,

C₅ = 56

Now,

If n = 9,

→ C = [tex]\frac{9!}{(4!\times 5!)}[/tex]

      = 126

and, If n = 10,

→ C = [tex]\frac{10!}{(5!\times 5!)}[/tex]

      = 252

and, If n = 11,

→ C = [tex]\frac{11!}{(6!\times 5!)}[/tex]

      = 462

Then perhaps the minimal number of individuals required to have more than 365 possible combinations seems to be 11.

Thus the response above is correct.

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