This website contains problems from math contests. Problems and corresponding tags were obtained from the Art of Problem Solving website.

Tags were heavily modified to better represent problems.

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Found problems: 14

2015 Purple Comet Problems, 6
Tags: divisibility tests , number theory
Find the least positive integer whose digits add to a multiple of 27 yet the number itself is not a multiple of 27. For example, 87999921 is one such number.

2018 Iran MO (1st Round), 3
Tags: divisibility tests , number theory
How many $8$-digit numbers in base $4$ formed of the digits $1,2, 3$ are divisible by $3$?

2014 AMC 12/AHSME, 23
Tags: divisibility tests , induction , modular arithmetic , function , number theory
The fraction \[\dfrac1{99^2}=0.\overline{b_{n-1}b_{n-2}\ldots b_2b_1b_0},\] where $n$ is the length of the period of the repeating decimal expansion. What is the sum $b_0+b_1+\cdots+b_{n-1}$? $\textbf{(A) }874\qquad \textbf{(B) }883\qquad \textbf{(C) }887\qquad \textbf{(D) }891\qquad \textbf{(E) }892\qquad$

2010 AIME Problems, 1
Tags: divisibility tests , modular arithmetic , number theory
Let $ N$ be the greatest integer multiple of $ 36$ all of whose digits are even and no two of whose digits are the same. Find the remainder when $ N$ is divided by $ 1000$.

2007 Middle European Mathematical Olympiad, 4
Tags: polynomial , divisibility tests , modular arithmetic , algebra , number theory
Find all positive integers $ k$ with the following property: There exists an integer $ a$ so that $ (a\plus{}k)^{3}\minus{}a^{3}$ is a multiple of $ 2007$.

2006 Iran Team Selection Test, 4
Tags: number theory , algebra , divisibility tests , polynomial
Let $n$ be a fixed natural number. Find all $n$ tuples of natural pairwise distinct and coprime numbers like $a_1,a_2,\ldots,a_n$ such that for $1\leq i\leq n$ we have \[ a_1+a_2+\ldots+a_n|a_1^i+a_2^i+\ldots+a_n^i \]

1965 Czech and Slovak Olympiad III A, 1
Tags: divisible , divisibility tests , number theory
Show that the number $5^{2n+1}2^{n+2}+3^{n+2}2^{2n+1}$ is divisible by $19$ for every non-negative integer $n$.

1989 Canada National Olympiad, 3
Tags: logarithm , number theory , algebra , binomial theorem , divisibility tests , modular arithmetic
Define $ \{ a_n \}_{n\equal{}1}$ as follows: $ a_1 \equal{} 1989^{1989}; \ a_n, n > 1,$ is the sum of the digits of $ a_{n\minus{}1}$. What is the value of $ a_5$?

2008 Baltic Way, 12
Tags: combinatorics , function , modular arithmetic , divisibility tests , graph theory , number theory
In a school class with $ 3n$ children, any two children make a common present to exactly one other child. Prove that for all odd $ n$ it is possible that the following holds: For any three children $ A$, $ B$ and $ C$ in the class, if $ A$ and $ B$ make a present to $ C$ then $ A$ and $ C$ make a present to $ B$.

2012 India National Olympiad, 2
Tags: prime number , divisibility tests , number theory
Let $p_1<p_2<p_3<p_4$ and $q_1<q_2<q_3<q_4$ be two sets of prime numbers, such that $p_4 - p_1 = 8$ and $q_4 - q_1= 8$. Suppose $p_1 > 5$ and $q_1>5$. Prove that $30$ divides $p_1 - q_1$.

2006 Iran Team Selection Test, 4
Tags: algebra , divisibility tests , number theory , polynomial
Let $n$ be a fixed natural number. Find all $n$ tuples of natural pairwise distinct and coprime numbers like $a_1,a_2,\ldots,a_n$ such that for $1\leq i\leq n$ we have \[ a_1+a_2+\ldots+a_n|a_1^i+a_2^i+\ldots+a_n^i \]

2009 Indonesia MO, 1
Tags: modular arithmetic , algebra , polynomial , quadratic , divisibility tests , number theory
Find all positive integers $ n\in\{1,2,3,\ldots,2009\}$ such that \[ 4n^6 \plus{} n^3 \plus{} 5\] is divisible by $ 7$.

2014 Lusophon Mathematical Olympiad, 5
Tags: number theory , divisibility tests
Find all quadruples of positive integers $(k,a,b,c)$ such that $2^k=a!+b!+c!$ and $a\geq b\geq c$.

1984 AIME Problems, 2
Tags: divisibility tests , number theory
The integer $n$ is the smallest positive multiple of 15 such that every digit of $n$ is either 8 or 0. Compute $\frac{n}{15}$.