CombinatoricsDifficulty 6.1National OlympiadProve itUnited 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 N be the number of subsets of the 16 chairs that could be selected. Find the remainder when N 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 q pairs PP and 8−2q Ps not adjacent to another P, where 0≤q≤4. There are 9 spaces before, after, or between the 8 Es, and PPs will be placed in q of those 9 spaces, while Ps will be placed in 8−2q of the remaining 9−q spaces. Hence the number of arrangements for a given value of q is (q9)(8−2q9−q), and the total number of ways to select chairs is q=0∑4(q9)(8−2q9−q)=(09)(89)+(19)(68)+(29)(47)+(39)(26)+(49)(05)=1⋅9+9⋅28+36⋅35+84⋅15+126⋅1=9+252+1260+1260+126=2907. The requested remainder is 907.
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Source: MathNet,
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