MathDB

Problems(2)

4x1, 2 3x1, 3 2x1, 4 1x1 pieces

Source: XVI May Olympiad (Olimpiada de Mayo) 2010 L2 P5

9/19/2022
You have the following pieces: one 4×14\times 1 rectangle, two 3×13\times 1 rectangles, three 2×12\times 1 rectangles, and four 1×11\times 1 squares. Ariel and Bernardo play the following game on a board of n×nn\times n, where nn is a number that Ariel chooses. In each move, Bernardo receives a piece RR from Ariel. Next, Bernardo analyzes if he can place RR on the board so that it has no points in common with any of the previously placed pieces (not even a common vertex). If there is such a location for RR, Bernardo must choose one of them and place RR. The game stops if it is impossible to place RR in the way explained, and Bernardo wins. Ariel wins only if all 1010 pieces have been placed on the board. a) Suppose Ariel gives Bernardo the pieces in decreasing order of size. What is the smallest n that guarantees Ariel victory? b) For the nn found in a), if Bernardo receives the pieces in increasing order of size, is Ariel guaranteed victory?
Note: Each piece must cover exactly a number of unit squares on the board equal to its own size. The sides of the pieces can coincide with parts of the edge of the board.
combinatorics
red points on 24 intersections of 2x7 grid

Source: XVΙ May Olympiad (Olimpiada de Mayo) 2010 L1 P5

9/22/2022
In a 2×7 2\times 7 board gridded in 1×11\times 1 squares, the 2424 points that are vertices of the squares are considered. https://cdn.artofproblemsolving.com/attachments/9/e/841f11ef9d6fc27cdbe7c91bab6d52d12180e8.gif Juan and Matías play on this board. Juan paints red the same number of points on each of the three horizontal lines. If Matthias can choose three red dots that are vertices of an acute triangle, Matthias wins the game. What is the maximum number of dots Juan can color in to make sure Matías doesn't win? (For the number found, give an example of coloring that prevents Matías from winning and justify why if the number is greater, Matías can always win.)
combinatoricsColoring