IMO 1986 LL BEL6
In an urn there are one ball marked 1, two balls marked 2, and
IMO 1986 LL BEL6
Origin: BEL
Problem
In an urn there are one ball marked 1, two balls marked 2, and so on, up to n balls marked n. Two balls are randomly drawn without replacement. Find the probability that the two balls are assigned the same number.