# Homework Help: Permutation and combination

1. Mar 23, 2012

### nik jain

1. The problem statement, all variables and given/known data
Let Pn denotes the no. of ways of selecting 3 people out of 'n' sitting in a row if no two of them are
consecutive and Qn is the corresponding figure when they are in circle .
If Pn - Qn = 6 , then 'n' is equal to:

3. The attempt at a solution

Pn = (n-2) C 3
Qn = (n-3) C 3
But I am getting wrong answer.
Please tell which one is wrong Pn or Qn .

2. Mar 23, 2012

### tiny-tim

hi nik jain!

(try using the X2 and X2 buttons just above the Reply box )
i'm mystified …

why do you think that either of these is correct?

but anyway you don't need to find Pn or Qn, you only need the difference

what selections are in Pn but not in Qn ?