If $70 + m + n - 20 = 80$ and $m = 2n$, then what is the value of $n$?
This problem involves two equations and two variables, so I will try to use substitution or elimination to solve for one of them. Since the second equation already expresses $m$ in terms of $n$, I will substitute that into the first equation and get $70 + 2n + n - 20 = 80$. Simplifying this equation, I get $3n + 50 = 80$. Subtracting $50$ from both sides, I get $3n = 30$. Dividing both sides by $3$, I get $n = 10$.

