jmc

After converting everything to base 10, we are able to solve for $△$. We get $$\begin{array}{rl}4{△}_9& =△0_7\Rightarrow \\ 4\cdot 9^1+△\cdot 9^0& =△\cdot 7^1+0\cdot 7^0\Rightarrow \\ 36+△& =7\cdot △\Rightarrow \\ 36& =6\cdot △\Rightarrow \\ 6& =△.\end{array}$$Now we can solve for $n$, which equals $46_9=60_7=\boxed{42}$.