All the squares of a \(9 \times 9\) chessboard were coloured black and white in a traditional way, such that the corner squares are all white. In each move, you can choose two neighbouring squares and change both of their colours – black to white and white to black. Can you reach a chessboard that is all black in this way?
a) A rook in chess can move any number of squares in the same row or column. Let’s invent a new figure, a “little rook” that can only move one square in each of these directions. If we start with a “little rook” in the bottom right corner of an \(8 \times 8\) chessboard, can we make it to the opposite corner while visiting each square exactly once?
b) A king in chess moves like the “little rook”, but he can also move one square along a diagonal. Can we do the same task with a king?
There are \(15\) lightbulbs in a row, they are all switched off. We can pick any three of them and change their state. Can we repeat this operation an even number of times such that at the end all the lightbulbs are on?
Are there such integer numbers \(a,b,c,d,e,f\) that the numbers \[a-b,\ b-c,\ c-d,\ d-e,\ e-f,\ f-a\] are consecutive integers? (Note: they obviously can be negative.)
There are numbers \(1,2,3,4,5,6,7,8,9\) and \(10\) written on a board. Each time you make a “move” you can erase three of the remaining numbers, \(a,b\) and \(c\), and replace them with the numbers \(2a+b\), \(2b+c\) and \(2c+b\). The goal is to make all the \(10\) numbers left on the board equal. Is it possible?
On a certain island there are 17 grey, 15 brown and 13 crimson chameleons. If two chameleons of different colours meet, both of them change to the third colour. No other colour changes are allowed. Is it possible that after a few such colour transitions all the chameleons have the same colour?
Sixteen lightbulbs are arranged in a \(4 \times 4\) grid. Some are on, some are off. You are allowed to change the state of all the bulbs in a column, in a row, or along any diagonal (note: there are 14 diagonals in total!). Is it possible to go from the arrangement in the left to the one on the right by repeating this operation?
There are numbers from \(1\) to \(2018\) written on a board. In one go, we can erase two numbers and replace them with an absolute value of their difference. Can we achieve a sequence consisting only of several numbers \(0\) after some number of moves?
The distance from school to the monument in the town centre is \(4.2\) km and the distance from Anna’s house to school is \(0.7\) km. Given that the distance from Anna’s house to the monument is an integer number of kilometres, what is this distance?
Tom and his grandma live on the same side of a straight river. Tom wants to visit his grandma, but also wants to stop by the river and fill his bottle with water. What is the shortest path that starts at his house, touches the river and ends at his grandma’s house?