It is known that among several banknotes of pairwise distinct face values (which are positive integers) there are exactly $N{}$ fakes. In a single test, a detector determines the sum of the face values of all real banknotes in an arbitrary set we have selected. Prove that by using the detector $N{}$ times, all fake banknotes can be identified, if a) $N=2$ and b) $N=3$. [i]Proposed by S. Tokarev[/i]

A polygon is called concave if it has at least one angle strictly greater than $180^{\circ}$. What is the maximum number of symmetries that an 11-sided concave polygon can have? (A [i]symmetry[/i] of a polygon is a way to rotate or reflect the plane that leaves the polygon unchanged.)

The decimal representation of all integers from $1$ to an arbitrary integer $n$ are written one after another as such: $$123... 91011... 99100... (n).$$ Does there exist $n$ such that each of the digits $0,1,2,...,9$ appears the same number of times in the given sequence? (A Andzans)

Let $x$ be the answer to this question. Find the value of $2017 - 2016x$. [i]Proposed by Michael Tang[/i]

Consider the digits $1, 2, 3, 4, 5, 6, 7$. (a) Determine the number of seven-digit numbers with distinct digits that can be constructed using the digits above. (b) If we place all of these seven-digit numbers in increasing order, find the seven-digit number which appears in the $2022^{\text{th}}$ position.

There are $2^n$ words of length $n$ over the alphabet $\{0, 1\}$. Prove that the following algorithm generates the sequence $w_0, w_1, \ldots, w_{2^n-1}$ of all these words such that any two consecutive words differ in exactly one digit. (1) $w_0 = 00 \ldots 0$ ($n$ zeros). (2) Suppose $w_{m-1} = a_1a_2 \ldots a_n,\quad a_i \in \{0, 1\}$. Let $e(m)$ be the exponent of $2$ in the representation of $n$ as a product of primes, and let $j = 1 + e(m)$. Replace the digit $a_j$ in the word $w_{m-1}$ by $1 - a_j$. The obtained word is $w_m$.

Find values of the parameter $u$ for which the expression \[y = \frac{ \tan(x-u) + \tan(x) + \tan(x+u)}{ \tan(x-u)\tan(x)\tan(x+u)}\] does not depend on $x.$

Three vertices of a cube are $P=(7,12,10),$ $Q=(8,8,1),$ and $R=(11,3,9).$ What is the surface area of the cube?

Find all pairs of natural numbers $(p, n)$ with $p$ prime such that $p^6+p^5+n^3+n=n^5+n^2$.

Let $p$ be a prime and let $f(x)$ be a polynomial of degree $d$ with integer coefficients. Assume that the numbers $f(1),f(2),\dots,f(p)$ leave exactly $k$ distinct remainders when divided by $p$, and $1<k<p$. Prove that \[ \frac{p-1}{d}\leq k-1\leq (p-1)\left(1-\frac1d \right) .\] [i] Dániel Domán, Gauls Károlyi, and Emil Kiss [/i]

Twelve 1-Euro-coins are laid flat on a table, such that their midpoints form a regular $12$-gon. Adjacent coins are tangent to each other. Prove that it is possible to put another seven such coins into the interior of the ring of the twelve coins.

How many pairs of integers $a$ and $b$ are there such that $a$ and $b$ are between $1$ and $42$ and $a^9 = b^7 \mod 43$?

Find all integers $m$ and $n$ such that $2m^2 +n^2 = 2mn+3n$

Let $ABC$ be a triangle and let $M$ be the midpoint of the side $AB$. Let $BD$ be the interior angle bisector of $\angle ABC$, $D\in AC$. Prove that if $MD \perp BD$ then $AB=3BC$.

Let $x, y, p, n, k$ be positive integers such that $$x^n + y^n = p^k.$$ Prove that if $n > 1$ is odd, and $p$ is an odd prime, then $n$ is a power of $p$.

Let $n$ be a given even number, $a_1,a_2,\cdots,a_n$ be non-negative real numbers such that $a_1+a_2+\cdots+a_n=1.$ Find the maximum possible value of $\sum_{1\le i<j\le n}\min\{(i-j)^2,(n+i-j)^2\}a_ia_j .$

A board $n \times n$ is divided into $n^2$ unit squares and a number is written in each unit square. Such a board is called [i] interesting[/i] if the following conditions hold: $\circ$ In all unit squares below the main diagonal, the number $0$ is written; $\circ$ Positive integers are written in all other unit squares. $\circ$ When we look at the sums in all $n$ rows, and the sums in all $n$ columns, those $2n$ numbers are actually the numbers $1,2,...,2n$ (not necessarily in that order). $a)$ Determine the largest number that can appear in a $6 \times 6$ [i]interesting[/i] board. $b)$ Prove that there is no [i]interesting[/i] board of dimensions $7\times 7$.

In the triangle $ABC$, the bisector of angle $\angle B$ meets $AC$ at $D$ and the bisector of angle $\angle C$ meets $AB$ at $E$. The bisectors meet each other at $O$. Furthermore, $OD = OE$. Prove that either $ABC$ is isosceles or $\angle BAC = 60^\circ$.

Tyrone had $ 97$ marbles and Eric had $ 11$ marbles. Tyrone then gave some of his marbles to Eric so that Tyrone ended with twice as many marbles as Eric. How many marbles did Tyrone give to Eric? $ \textbf{(A)}\ 3 \qquad \textbf{(B)}\ 13 \qquad \textbf{(C)}\ 18 \qquad \textbf{(D)}\ 25 \qquad \textbf{(E)}\ 29$

Let $ABC$ be a triangle, $\Gamma$ its circumcircle and $D$ the midpoint of the arc $AC$ of $\Gamma$ that does not contain $B$. If $O$ is the center of $\Gamma$ and I is the incenter of $ABC$, prove that $OI$ is perpendicular to $BD$ if and only if $AB + BC = 2AC$.

Using only angle with angle $\frac{\pi}{7}$ and a ruler, constuct angle $\frac{\pi}{14}$

Let $p \ge 5$ be a prime number. For a positive integer $k$, let $R(k)$ be the remainder when $k$ is divided by $p$, with $0 \le R(k) \le p-1$. Determine all positive integers $a < p$ such that, for every $m = 1, 2, \cdots, p-1$, $$ m + R(ma) > a. $$

The integers from $1$ to $25,$ inclusive, are randomly placed into a $5$ by $5$ grid such that in each row, the numbers are increasing from left to right. If the columns from left to right are numbered $1,2,3,4,$ and $5,$ then the expected column number of the entry $23$ can be written as $\tfrac{a}{b}$ where $a$ and $b$ are relatively prime positive integers. Find $a+b.$

$\definecolor{A}{RGB}{70,80,0}\color{A}\fbox{C4.}$ Show that for any positive integer $n \ge 3$ and some subset of $\lbrace{1, 2, . . . , n}\rbrace$ with size more than $\frac{n}2 + 1$, there exist three distinct elements $a, b, c$ in the subset such that $$\definecolor{A}{RGB}{255,70,255}\color{A} (ab)^2 + (bc)^2 + (ca)^2$$is a perfect square. [i]Proposed by [/i][b][color=#419DAB]ltf0501[/color][/b]. [color=#3D9186]#1736[/color]

The exchange rates of the Dollar and the German mark during the week changed as follows: $\begin{tabular}{|l|l|l|} \hline & Dollar & Mark \\ \hline Monday & 4000 rub. & 2500 rub. \\ \hline Tuesday & 4500 rub. & 2800 rub.\\ \hline Wednesday & 5000 rub. & 2500 rub.\\ \hline Thursday & 4500 rub. & 3000 rub.\\ \hline Friday & 4000 rub. & 2500 rub.\\ \hline Saturday & 4500 rub. & 3000 rub.\\ \hline \end{tabular}$ What percentage was the maximum possible increase in capital this week by playing on changes in the exchange rates of these currencies? (The initial capital was in rubles. The final capital should also be in rubles. During the week, the available money can be distributed as desired into rubles, dollars and marks. The selling and purchasing rates are considered the same.)