olymmath

ISL 1988 Problem 81

    There are n≥3n \geq 3 job openings at a factory, ranked 11 to nn in order of increasing pay. There are nn job applicants, ranked from 11 to nn in order of increasing ability. Applicant ii is qualified for job jj if and only if i≥j.i \geq j. The applicants arrive one at a time in random order. Each in turn is hired to the highest-ranking job for which he or she is qualified AND which is lower in rank than any job already filled. (Under these rules, job 11 is always filled, and hiring terminates thereafter.) Show that applicants nn and n−1n - 1 have the same probability of being hired.

    Dataset text AI-MO AoPS. Quoted with source.

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