There are two people on facebook with same number of friends.

So we let X= set with at least 2 people.

Let Y= Possible number of friends, person in X can have.

Y is a subset {0,1,2,...,n-1}

define f:X->Y,

where f(x) = number of friends X has. (WTS: | Y | is less than or equal to n.

so f(x_0)=0 for some x_0 in x means someone has no friends.

Then this means, no one can have n-1 friends, (i.e. friends with everyone else). I am stuck here and not sure where to go.