2009 AIME II Problems/Problem 6

Problem

Let be the number of five-element subsets that can be chosen from the set of the first natural numbers so that at least two of the five numbers are consecutive. Find the remainder when is divided by .

Solution

We can use complementary counting. We can choose a five-element subset in ways. We will now count those where no two numbers are consecutive. We will show a bijection between this set, and the set of 10-element strings that contain 5 s and 5 s, thereby showing that there are such sets.

Given a five-element subset of in which no two numbers are consecutive, we can start by writing down a string of length 14, in which the -th character is if and otherwise. Now we got a string with 5 s and 9 s. As no two numbers were consecutive, we know that in our string no two s are consecutive. We can now remove exactly one from between each pair of s to get a string with 5 s and 5 s. And clearly this is a bijection, as from each string with 5 s and 5 s we can reconstruct one original set by reversing the construction.

Hence we have , and the answer is .