CP #13

 From GMATClub's Probability chapter:


Q: There are 5 chairs. Bob and Rachel want to sit such that Bob is always left to Rachel. How many ways it can be done ?
Solution: Because of symmetry, the number of ways that Bob is left to Rachel is exactly 1/2 of all possible ways:
N = 1/2 * 5P2 = 10

Why half?