IMO 2018 Shortlist C2
Queenie and Horst play a game on a 20ˆ20 hessboard. In the beginning the board is empty. In every turn, Horst pla es a b...
Category: Combinatorics
Problem
Queenie and Horst play a game on a 20ˆ20 hessboard. In the beginning the board is empty. In every turn, Horst pla es a bla k knight on an empty square in su h a way that his new knight does not atta k any previous knights. Then Queenie pla es a white queen on an empty square. The game gets nished when somebody annot move. Find the maximal positive K su h that, regardless of the strategy of Queenie, Horst an put at least K knights on the board. (Armenia)