From all possible permutations from $(a_1,a_2,...,a_n)$ from the set $\{1,2,..,n\}$, $n\geq 1$, consider the sets that satisfies the $2(a_1+a_2+...+a_m)$ is divisible by $m$, for every $m=1,2,...,n$. Find the total number of permutations.

Let $n \ge 2$ be a positive integer. Each square of an $n\times n$ board is coloured red or blue. We put dominoes on the board, each covering two squares of the board. A domino is called [i]even [/i] if it lies on two red or two blue squares and [i]colourful [/i] if it lies on a red and a blue square. Find the largest positive integer $k$ having the following property: regardless of how the red/blue-colouring of the board is done, it is always possible to put $k$ non-overlapping dominoes on the board that are either all [i]even [/i] or all [i]colourful[/i].

Every positive integer greater than $1000$ is colored in red or blue, such that the product of any two distinct red numbers is blue. Is it possible to happen that no two blue numbers have difference $1$?

Let $S$ be the set of integers modulo $2020$. Suppose that $a_1,a_2,...,a_{2020},b_1,b_2,...,b_{2020}, c$ are arbitrary elements of $S$. For any $x_1,x_2,...,x_{2020}\in S$, define $f(x_1,x_2,...,x_{2020})$ to be the $2020$-tuple whose $i$th coordinate is $x_{i-2} + a_i x_{2019} + b_ix_{2020} + cx_i$, where we set $x_{-1}=x_0=0$. Let $m$ be the smallest positive integer such that, for some values of $a_1,a_2,...,a_{2020},b_1,b_2,...,b_{2020}, c$, we have, for all $x_1,x_2,...,x_{2020}\in S$, that $f^m (x_1, x_2, ..., x_{2020} ) = (0,0,...,0)$ . For this value of $m$, there are exactly $n$ choices of the tuple $(a_1,a_2,...,a_{2020},b_1,b_2,...,b_{2020},c)$ such that, for all $x_1,x_2,...,x_{2020}\in S$, $f^m (x_1, x_2, ..., x_{2020} ) = (0,0,...,0)$. Compute $100m+n$. [i]Proposed by Vincent Huang[/i]

The largest prime factor of $199^4 + 4$ has four digits. Compute the second largest prime factor.

Let $x_1, x_2, \dots, x_n$ be different real numbers. Prove that \[\sum_{1 \leqslant i \leqslant n} \prod_{j \neq i} \frac{1-x_{i} x_{j}}{x_{i}-x_{j}}=\left\{\begin{array}{ll} 0, & \text { if } n \text { is even; } \\ 1, & \text { if } n \text { is odd. } \end{array}\right.\]

For a finite nonempty set $A$ of positive integers, $A =\{a_1, a_2,\dots , a_n\}$, we say the calamitous complement of A is the set of all positive integers $k$ for which there do not exist nonnegative integers $w_1, w_2, \dots , w_n$ with $k = a_1w_1 + a_2w_2 +\dots + a_nw_n.$ The calamitous complement of $A$ is denoted $cc(A)$. For example, $cc(\{5, 6, 9\}) = \{1, 2, 3, 4, 7, 8, 13\}$. Find all pairs of positive integers $a, b$ with $1 < a < b$ for which there exists a set $G$ satisfying all of the following properties: 1. $G$ is a set of at most three positive integers, 2. $cc(\{a, b\})$ and $cc(G)$ are both finite sets, and 3. $cc(G) = cc(\{a, b\})\cup \{m\}$ for some $m$ not in $cc(\{a, b\})$.

Two opposite sides of a rectangle are each divided into $n$ congruent segments, and the endpoints of one segment are joined to the center to form triangle $A$. The other sides are each divided into $m$ congruent segments, and the endpoints of one of these segments are joined to the center to form triangle $B$. [See figure for $n = 5, m = 7$.] What is the ratio of the area of triangle $A$ to the area of triangle $B$? [asy] int i; for(i=0; i<8; i=i+1) { dot((i,0)^^(i,5)); } for(i=1; i<5; i=i+1) { dot((0,i)^^(7,i)); } draw(origin--(7,0)--(7,5)--(0,5)--cycle, linewidth(0.8)); pair P=(3.5, 2.5); draw((0,4)--P--(0,3)^^(2,0)--P--(3,0)); label("$B$", (2.3,0), NE); label("$A$", (0,3.7), SE);[/asy] $\text{(A)} \ 1 \qquad \text{(B)} \ m/n \qquad \text{(C)} \ n/m \qquad \text{(D)} \ 2m/n \qquad \text{(E)} \ 2n/m$

The average age of $33$ fifth-graders is $11$. The average age of $55$ of their parents is $33$. What is the average age of all of these parents and fifth-graders? $\textbf{(A) }22\qquad\textbf{(B) }23.25\qquad\textbf{(C) }24.75\qquad\textbf{(D) }26.25\qquad\textbf{(E) }28$

$a_n,b_n,c_n$ are three sequences of positive integers satisfying $$\prod_{d|n}a_d=2^n-1,\prod_{d|n}b_d=\frac{3^n-1}{2},\prod_{d|n}c_d=\gcd(2^n-1,\frac{3^n-1}{2})$$ for all $n\in \mathbb{N}$. Prove that $\gcd(a_n,b_n)|c_n$ for all $n\in \mathbb{N}$

Call an ordered triple $(a, b, c)$ of integers feral if $b -a, c - a$ and $c - b$ are all prime. Find the number of feral triples where $1 \le a < b < c \le 20$.

Consider sequences of positive real numbers of the form $ x,2000,y,...,$ in which every term after the first is 1 less than the product of its two immediate neighbors. For how many different values of $ x$ does the term 2001 appear somewhere in the sequence? $ \textbf{(A)} \ 1 \qquad \textbf{(B)} \ 2 \qquad \textbf{(C)} \ 3 \qquad \textbf{(D)} \ 4 \qquad \textbf{(E)} \ \text{more than 4}$

$A$ and $B$ are $2\times 2$ real valued matrices satisfying $$\det A = \det B = 1,\quad \text{tr}(A)>2,\quad \text{tr}(B)>2,\quad \text{tr}(ABA^{-1}B^{-1}) = 2$$ Prove that $A$ and $B$ have a common eigenvector.

Several pupils with different heights are standing in a row. If they were arranged according to their heights, such that the highest would stand on the right, then each pupil would move for at most 8 positions. Prove that every pupil has no more than 8 pupils lower then him on his right.

How many real numbers $x$ satisfy the equation $3^{2x+2}-3^{x+3}-3^{x}+3=0$? $\textbf {(A) } 0 \qquad \textbf {(B) } 1 \qquad \textbf {(C) } 2 \qquad \textbf {(D) } 3 \qquad \textbf {(E) } 4$

We say that a positive integer $n$ is $m$[i]-expressible[/i] if it is possible to get $n$ from some $m$ digits and the six operations $+,-,\times,\div$, exponentiation $^\wedge$, and concatenation $\oplus$. For example, $5625$ is $3$-expressible (in two ways): both $5\oplus (5^\wedge 4)$ and $(7\oplus 5)^\wedge 2$ yield $5625$. Does there exist a positive integer $N$ such that all positive integers with $N$ digits are $(N-1)$-expressible? [i]Proposed by Krit Boonsiriseth[/i]

Tony's favorite "sport" is a spectator event known as the $\textit{Super Mega Ultra Galactic Thumbwrestling Championship}$ (SMUG TWC). During the $2008$ SMUG TWC, $2008$ professional thumb-wrestlers who have dedicated their lives to earning lithe, powerful thumbs, compute to earn the highest title of $\textit{Thumbzilla}$. The SMUG TWC is designed so that, in the end, any set of three participants can share a banana split while telling FOX$^\text{TM}$ television reporters about a bout between some pair of the three contestants. Given that there are exactly two contestants in each bout, let $m$ be the minimum bumber of bouts necessary to complete the SMUG TWC (so that the contestants can enjoy their banana splits and chat with reporters). Compute $m$.

For the real pairs $(x,y)$ satisfying the equation $x^2 + y^2 + 2x - 6y = 6$, which of the following cannot be equal to $(x-1)^2 + (y-2)^2$? $ \textbf{(A)}\ 2 \qquad\textbf{(B)}\ 9 \qquad\textbf{(C)}\ 16 \qquad\textbf{(D)}\ 23 \qquad\textbf{(E)}\ 30 $

Which of the following is not equal to $\dfrac{5}{4}$? $\text{(A)}\ \dfrac{10}{8} \qquad \text{(B)}\ 1\dfrac{1}{4} \qquad \text{(C)}\ 1\dfrac{3}{12} \qquad \text{(D)}\ 1\dfrac{1}{5} \qquad \text{(E)}\ 1\dfrac{10}{40}$

Given a positive integer $n$ and $I = \{1, 2,..., k\}$ with $k$ is a positive integer. Given positive integers $a_1, a_2, ..., a_k$ such that for all $i \in I$: $1 \le a_i \le n$ and $$\sum_{i=1}^k a_i \ge 2(n!).$$ Show that there exists $J \subseteq I$ such that $$n! + 1 \ge \sum_{j \in J}a_j >\sqrt {n! + (n - 1)n}$$

A token is placed in the leftmost square in a strip of four squares. In each move, you are allowed to move the token left or right along the strip by sliding it a single square, provided that the token stays on the strip. In how many ways can the token be moved so that after exactly $15$ moves, it is in the rightmost square of the strip?

How many angles $\theta$ with $0\le\theta\le2\pi$ satisfy $\log(\sin(3\theta))+\log(\cos(2\theta))=0$? $ \textbf{(A) }0 \qquad \textbf{(B) }1 \qquad \textbf{(C) }2 \qquad \textbf{(D) }3 \qquad \textbf{(E) }4 \qquad $

Let $ABC$ be a triangle, and let $BE, CF$ be the altitudes. Let $\ell$ be a line passing through $A$. Suppose $\ell$ intersect $BE$ at $P$, and $\ell$ intersect $CF$ at $Q$. Prove that: i) If $\ell$ is the $A$-median, then circles $(APF)$ and $(AQE)$ are tangent. ii) If $\ell$ is the inner $A$-angle bisector, suppose $(APF)$ intersect $(AQE)$ again at $R$, then $AR$ is perpendicular to $\ell$.

Are there $10$ consecutive positive integers, such that if we consider the digits that appear in the decimal representations of those numbers as a multiset, every digit appears the same number of times in this multiset?

For a positive integer $n$ let $S(n)$ be the sum of digits in the decimal representation of $n$. Any positive integer obtained by removing several (at least one) digits from the right-hand end of the decimal representation of $n$ is called a [i]stump[/i] of $n$. Let $T(n)$ be the sum of all stumps of $n$. Prove that $n=S(n)+9T(n)$.