GCD of the elements of a set 
Let $S$ be the set of numbers of the form
  $n(n + 1)(n + 2)(n + 3)(n + 4),$
  where $n$ is any positive integer. The first few terms of $S$ are
  \begin{align*}
1 \cdot 2 \cdot 3 \cdot 4 \cdot 5 &= 120, \\
2 \cdot 3 \cdot 4 \cdot 5 \cdot 6 &= 720, \\
3 \cdot 4 \cdot 5 \cdot 6 \cdot 7 &= 2520,
\end{align*}
  and so on. What is the GCD of the elements of $S$?

Every other number should be even, so $n(n + 1)(n + 2)(n + 3)(n + 4)$ has at least a factor of $2^2=4$ (or $2^3=8$ when $n, n+2$ and $n+4$ are all even). Every third number is divisible by $3$, so $n(n + 1)(n + 2)(n + 3)(n + 4)$ is at least divisible by $3$. Every 5 number is divisible by $5$, so $n(n + 1)(n + 2)(n + 3)(n + 4)$ is divisible by $5$ as well. So I put $3\times 4\times 5 = 60$ as the answer, which is wrong. But I don't think numbers above $5$ are guaranteed to be a factor of $n(n + 1)(n + 2)(n + 3)(n + 4)$...

For $n\ge 1$, the number $$\frac{n(n+1)(n+2)(n+3)(n+4)}{120}$$ is just the binomial coefficient $\binom{n+4}{5}$ which is always an integer. Hence all the numbers are divisible by $120$. 
Because $120$ is the smallest , the $\gcd$ of all the numbers is $120$
