2014 AIME II Problems/Problem 9

Solution

Starting with small cases, we see that four chairs give 4 + 1 = 5, five chairs give 5 + 5 + 1 = 11, and six chairs give 6 + 6 + 6 + 6 + 1 = 25. Thus, I claim that n chairs give , as confirmed above. This claim can be verified by PIE: there are ways to arrange 3 adjacent chairs, but then we subtract ways to arrange 4. Finally, we add 1 to account for all chairs case (the other cases have been taken care of). Thus, for n = 10 we get .