2017 AMC 12B Problem 25

Below is the professionally curated solution for Problem 25 of the 2017 AMC 12B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2017 AMC 12B solutions, or check the answer key.

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Concepts:double countingdivisibilityChinese Remainder Theorem

Difficulty rating: 2650

25.

A set of nn people participate in an online video basketball tournament. Each person may be a member of any number of 55-player teams, but no two teams may have exactly the same 55 members. The site statistics show a curious fact: The average, over all subsets of size 99 of the set of nn participants, of the number of complete teams whose members are among those 99 people is equal to the reciprocal of the average, over all subsets of size 88 of the set of nn participants, of the number of complete teams whose members are among those 88 people. How many values n,n, 9n2017,9 \le n \le 2017, can be the number of participants?

477477

482482

487487

557557

562562

Solution:

Let TT be the number of teams. Summing over size-99 subsets counts each team (n54)\binom{n-5}{4} times and over size-88 subsets (n53)\binom{n-5}{3} times. The averages are (n54)T(n9)\dfrac{\binom{n-5}{4}T}{\binom n9} and (n53)T(n8);\dfrac{\binom{n-5}{3}T}{\binom n8}; setting the first equal to the reciprocal of the second and simplifying gives T=n(n1)(n2)(n3)(n4)253257.T = \frac{n(n-1)(n-2)(n-3)(n-4)}{2^5 \cdot 3^2 \cdot 5 \cdot 7}. We need this to be a positive integer with n9.n \ge 9. Let N=n(n1)(n2)(n3)(n4);N = n(n-1)(n-2)(n-3)(n-4); as a product of five consecutive integers, NN is always divisible by 5.5. Checking residues, 7N,7 \mid N, 9N,9 \mid N, and 32N32 \mid N each hold for a fixed set of residues, giving 578=2805 \cdot 7 \cdot 8 = 280 solutions modulo 1008.1008. So there are 560560 values in 1n2016;1 \le n \le 2016; removing n=1,2,3,4n = 1, 2, 3, 4 (which are below 99) and adding n=2017n = 2017 (since 20171(mod1008)2017 \equiv 1 \pmod{1008}) gives 5604+1=557560 - 4 + 1 = 557 valid values.

Thus, the correct answer is D.

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