PAMO 2017 Shortlist: Extrema of sum of numbers on a chessboard
Source: 2017 Pan-African Shortlist - C2
May 5, 2019
combinatorics
Problem Statement
On a chessboard, we put, in the lower left corner, a die whose faces are numbered from to . By convention, the sum of digits on two opposite side of the die equals . Adama wants to move the die to the diagonally opposite corner using the following rule: at each step, Adama can roll the die only on to its right side, or to its top side. We suppose that whenever the die lands on a square, the number on its bottom face is printed on the square. By the end of these operations, Adama wants to find the sum of the numbers appearing on the chessboard. What are the maximum and minimum possible values of this sum?