AlgebraDifficulty 5.0AIME, harderProve itUnited States
Problem: Find the smallest positive integer n such that, if there are initially n+1 townspeople and n goons, then the probability the townspeople win is less than 1%.
Solution
Solution: Answer: 6
By a similar inductive argument, the probability for a given n is pn=(2n+1)!!n!. Clearly this is decreasing in n. It is easy to see that p5=3⋅5⋅7⋅9⋅111⋅2⋅3⋅4⋅5=6938>0.01 and p6=136p5=693⋅1348<0.01 Hence the answer is n=6. Heuristically, pn+1≈21pn for each n, so arriving at these estimates for the correct answer of n is not difficult.
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.