Tick, Trick and Track have 20, 23 and 25 tickets respectively for the carousel at the fair in Duckburg. They agree that they will only ride all three together, for which they must each give up one of their tickets. Also, before a ride, if they want, they can redistribute tickets among themselves as many times as they want according to the following rule: If one has an even number of tickets, he can give half of his tickets to any of the other two. Can it happen that after any trip: (a) exactly one has no ticket left, (b) exactly two have no ticket left, (c) all tickets are given away?

Determine all triples $(x, y, z)$ of integers that satisfy the equation $x^2+ y^2+ z^2 - xy - yz - zx = 3$

Given two parallelograms $ABCD$ and $AECF$ with common diagonal $AC$, where $E$ and $F$ lie inside parallelogram $ABCD$. Show: The circumcircles of the triangles $AEB$, $BFC$, $CED$ and $DFA$ have one point in common.

Given a real number $\alpha$ in whose decimal representation $\alpha=0,a_1a_2a_3\dots$ each decimal digit $a_i$ $(i=1,2,3,\dots)$ is a prime number. The decimal digits are arranged along the path indicated by arrows in the accompanying figure, which can be thought of as continuing infinitely to the right and downward. For each $m\geq 1$, the decimal representation of a real number $z_m$ is formed by writing before the decimal point the digit 0 and after the decimal point the sequence of digits of the $m$-th row from the top read from left to right from the adjacent arrangement. In an analogous way, for all $n\geq 1$, the real numbers $s_n$ are formed with the digits of the $n$-th column from the left to be read from top to bottom. For example, $z_3=0,a_5a_6a_7a_{12}a_{23}a_{28}\dots$ and $s_2=0,a_2a_3a_6a_{15}a_{18}a_{35}\dots$. Show: (a) If $\alpha$ is rational, then all $z_m$ and all $s_n$ are rational. (b) The converse of the statement formulated in (a) is false.

Determine the greatest common divisor of the numbers $p^6-7p^2+6$ where $p$ runs through the prime numbers $p \ge 11$.

A hilly island has $2023$ lookouts. It is known that each of them is in line of sight with at least $42$ of the other lookouts. For any two distinct lookouts $X$ and $Y$ there is a positive integer $n$ and lookouts $A_1,A_2,\dots,A_{n+1}$ such that $A_1=X$ and $A_{n+1}=Y$ and $A_1$ is in line of sight with $A_2$, $A_2$ with $A_3$, $\dots$ and $A_n$ with $A_{n+1}$. The smallest such number $n$ is called the viewing distance of $X$ and $Y$. Determine the largest possible viewing distance that can exist between two lookouts under these conditions.

Let $ABC$ be a triangle with incenter $I$. Let $M_b$ and $M_a$ be the midpoints of $AC$ and $BC$, respectively. Let $B'$ be the point of intersection of lines $M_bI$ and $BC$, and let $A'$ be the point of intersection of lines $M_aI$ and $AC$. If triangles $ABC$ and $A'B'C$ have the same area, what are the possible values of $\angle ACB$?

Exactly $n$ chords (i.e. diagonals and edges) of a regular $2n$-gon are coloured red, satisfying the following two conditions: (1) Each of the $2n$ vertices occurs exactly once as the endpoint of a red chord. (2) No two red chords have the same length. For which positive integers $n \ge 2$ is this possible?