The other number is $72−n$.
Work Step by Step
Let us call the other number $g$. If the sum of $n$ and the other number, $g$, is $72$, we can represent this with the following equation: $g+n=72$. We are looking for $g$. In order to find it, we must subtract $n$ on both sides of the equation: $g=72−n$. Since the "other number" is $g$, the "other number" is $72−n$.