2012 preRMO p6, 5 letters to 5 houses , postman
Source:
June 16, 2019
combinatorics
Problem Statement
A postman has to deliver five letters to five different houses. Mischievously, he posts one letter through each door without looking to see if it is the correct address. In how many different ways could he do this so that exactly two of the five houses receive the correct letters?