Maths Olympiad Prep

Library / /7 of 7

Combinatorics Difficulty 6.1 National Olympiad Prove it United States

Sixteen chairs are arranged in a row. Eight people each select a chair in which to sit so that no person sits next to two other people. Let NN be the number of subsets of the 16 chairs that could be selected. Find the remainder when NN is divided by 1000.

Solution

The problem is equivalent to counting the rearrangements of PPPPPPPPEEEEEEEE (standing for people and empty seats) where no three Ps appear together. Suppose such an arrangement contains qq pairs PPPP and 82q8 - 2q Ps not adjacent to another P, where 0q40 \le q \le 4. There are 9 spaces before, after, or between the 8 Es, and PPPPs will be placed in qq of those 9 spaces, while Ps will be placed in 82q8 - 2q of the remaining 9q9 - q spaces. Hence the number of arrangements for a given value of qq is (9q)(9q82q)\binom{9}{q}\binom{9-q}{8-2q}, and the total number of ways to select chairs is
q=04(9q)(9q82q)=(90)(98)+(91)(86)+(92)(74)+(93)(62)+(94)(50)=19+928+3635+8415+1261=9+252+1260+1260+126=2907. \begin{aligned} \sum_{q=0}^{4} \binom{9}{q} \binom{9-q}{8-2q} &= \binom{9}{0} \binom{9}{8} + \binom{9}{1} \binom{8}{6} + \binom{9}{2} \binom{7}{4} + \binom{9}{3} \binom{6}{2} + \binom{9}{4} \binom{5}{0} \\ &= 1 \cdot 9 + 9 \cdot 28 + 36 \cdot 35 + 84 \cdot 15 + 126 \cdot 1 = 9 + 252 + 1260 + 1260 + 126 = 2907. \end{aligned}
The requested remainder is 907.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.