Olympiad Maths Prep

Track / Stage 3 / 210 of 260 #210 of 2000

Problem 210

AMC 10/12, early questions
Combinatorics Difficulty 3.8 Find the answer

Let mm be the number of five-element subsets that can be chosen from the set of the first 1414 natural numbers so that at least two of the five numbers are consecutive. Find the remainder when mm is divided by 10001000.

Official solution

We can use complementary counting. We can choose a five-element subset in (145){14\choose 5} 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 AAs and 5 BBs, thereby showing that there are (105){10\choose 5} such sets.
Given a five-element subset SS of {1,2,,14}\{1,2,\dots,14\} in which no two numbers are consecutive, we can start by writing down a string of length 14, in which the ii-th character is AA if iSi\in S and BB otherwise. Now we got a string with 5 AAs and 9 BBs. As no two numbers were consecutive, we know that in our string no two AAs are consecutive. We can now remove exactly one BB from between each pair of AAs to get a string with 5 AAs and 5 BBs. And clearly this is a bijection, as from each string with 5 AAs and 5 BBs we can reconstruct one original set by reversing the construction.
Hence we have m=(145)(105)=2002252=1750m = {14\choose 5} - {10\choose 5} = 2002 - 252 = 1750, and the answer is 1750mod1000=7501750 \bmod 1000 = \boxed{750}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.