Show that the number of people who ever lived and made an odd number of handshakes is even.
Is it possible to trace the lines in the figures below in such a way that you trace each line only once?
Can you draw 9 line segments in such a way that each segment crosses exactly 3 other segments?
Numbers \(1,2,\dots,20\) are written on a whiteboard. In one go Louise is allowed to wipe out any two numbers \(a\) and \(b\), and write their sum \(a+b\) instead. Louise enjoys erasing the numbers, and continues the procedure until only one number is left on the whiteboard. What number is it?
Three tablespoons of milk from a glass of milk are poured into a glass of tea, and the liquid is thoroughly mixed. Then three tablespoons of this mixture are poured back into the glass of milk. Which is greater now: the percentage of milk in the tea or the percentage of tea in the milk?
Louise has a chessboard \(8\times8\) without two opposite corners (see the picture), and 31 dominoes \(2\times1\). Can she tile the crippled chessboard with dominoes she got?
Numbers \(1,2,\dots,20\) are written on a whiteboard. In one go Louise is allowed to wipe out any two numbers \(a\) and \(b\), and write instead
(a) \(a+b-1\); (b) \(a\times b\).
As you already know, Louise enjoys erasing the numbers, and has fun until only one number is left on the whiteboard. What number is it?
There is a \(3 \times 3\) grid filled with zeros. Louise is allowed to add 1 to each small square inside any \(2\times2\) grid. Can she ever get the following table as a result of her actions?
Prove that a \(10\times10\) board cannot be covered by T-shaped tiles (shown below)
Zara has an \(8\times8\) chessboard, in the usual coloring. She can repaint all the squares of a row or column, i.e., all white squares become black, and all black squares become white. Can she get exactly one black square?