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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Difficulty rating: 2650
25.
A set of people participate in an online video basketball tournament. Each person may be a member of any number of -player teams, but no two teams may have exactly the same members. The site statistics show a curious fact: The average, over all subsets of size of the set of participants, of the number of complete teams whose members are among those people is equal to the reciprocal of the average, over all subsets of size of the set of participants, of the number of complete teams whose members are among those people. How many values can be the number of participants?
Solution:
Let be the number of teams. Summing over size- subsets counts each team times and over size- subsets times. The averages are and setting the first equal to the reciprocal of the second and simplifying gives We need this to be a positive integer with Let as a product of five consecutive integers, is always divisible by Checking residues, and each hold for a fixed set of residues, giving solutions modulo So there are values in removing (which are below ) and adding (since ) gives valid values.
Thus, the correct answer is D.
Problem 25 in Other Years
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