From a group 2 groups of 2-person groups are formed in 105 ways.How many persons are there in this group.
Answer is 7.But I don't know how.
If we from 2 group of 2 persons from 7 it makes 35.
C(5,1)C(7,1)
I thought different.
There are n persons.If we choose 1 person n-1 remains.
From n-1 we choose 1 person.
C(n-1,1)
n-2 remained. From n-2 we call one person and put him aside.
n-3 remained.From that we choose 1.
And
C(n-1,1)C(n-3,1)
I thought that way. I think it is wrong.
You divided by 2 because there are 2 groups. We don't want c,a and a,c at the same time. Am I right?