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236

For a positive integer $p$, define the positive integer $n$ to be $p$-safe if $n$ differs in absolute value by more than $2$ from all multiples of $p$. For example, the set of $10$-safe numbers is $\{ 3, 4, 5, 6, 7, 13, 14, 15, 16, 17, 23, \ldots\}$. Find the number of positive integers less than or equal to $10,000$ which are simultaneously $7$-safe, $11$-safe, and $13$-safe.


268

There are $N$ permutations $(a_{1}, a_{2}, ... , a_{30})$ of $1, 2, \ldots, 30$ such that for $m \in \left\{{2, 3, 5}\right\}$, $m$ divides $(a_{n+m} - a_{n})$ for all integers $n$ with $1 \leq n < n+m \leq 30$. Find $N$.


Farmer Hank has fewer than $100$ pigs on his farm. If he groups the pigs five to a pen, there are always three pigs left over. If he groups the pigs seven to a pen, there is always one pig left over. However, if he groups the pigs three to a pen, there are no pigs left over. What is the greatest number of pigs that Farmer Hank could have on his farm?

Solve $$\left\{ \begin{array}{rcl} x &\equiv 2 &\pmod{3}\\ x &\equiv 2 &\pmod{5}\\ x &\equiv -3 &\pmod{7}\\x &\equiv -2 &\pmod{13} \end{array}\right.$$


Solve $$\left\{ \begin{array}{rcl} 4x & \equiv 14 &\pmod{15}\\ 9x & \equiv 11 &\pmod{20}\\ \end{array}\right.$$


Find the least non-negative residue of $70! \pmod{5183}$.


Solve the system of congruence $$\left\{ \begin{array}{l} x\equiv 1\pmod{3}\\ x\equiv 2\pmod{5}\\ x\equiv 3\pmod{7} \end{array}  \right.$$


Solve the congruent system: $4x\equiv 2\pmod{6}$ and $3x\equiv 5\pmod{8}$.


Find the smallest positive integer $n$ such that $$\left\{  \begin{array}{l} n\equiv 1\pmod{3} \\ n\equiv 3\pmod{5} \\ n\equiv 5\pmod{7} \end{array} \right.$$


Let $n$ be a positive integer. Show that there exist $n$ consecutive integers each of which contains a divisor who is a square number greater than $1$.

Compute $3^{2017}\pmod{1000}$.


Find the last $4$ digits of $2018^{2019^{2020}}$.