# International Mathematics Competition for University Students 2021

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IMC 2021
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## IMC2021: Day 1, Problem 2

Problem 2. Let $\displaystyle n$ and $\displaystyle k$ be fixed positive integers, and let $\displaystyle a$ be an arbitrary non-negative integer. Choose a random $\displaystyle k$-element subset $\displaystyle X$ of $\displaystyle \{1,2,\ldots,k+a\}$ uniformly (i.e., all $\displaystyle k$-element subsets are chosen with the same probability) and, independently of $\displaystyle X$, choose a random $\displaystyle n$-element subset $\displaystyle Y$ of $\displaystyle \{1,\ldots,k+n+a\}$ uniformly.

Prove that the probability

$\displaystyle \mathsf{P}\Big(\min(Y)>\max(X)\Big)$

does not depend on $\displaystyle a$.

Fedor Petrov, St. Petersburg State University IMC1994 IMC1995 IMC1996 IMC1997 IMC1998 IMC1999 IMC2000 IMC2001 IMC2002 IMC2003 IMC2004 IMC2005 IMC2006 IMC2007 IMC2008 IMC2009 IMC2010 IMC2011 IMC2012 IMC2013 IMC2014 IMC2015 IMC2016 IMC2017 IMC2018 IMC2019 IMC2020 IMC 2021 