A production line at V$.$ J$.$ Sugumaran's machine shop has three stations. The first station can process a unit in $9$ minutes. The second station has two identical machines

process a unit in $12$ minutes $($each unit only needs to be processed on one of the two machines$).$ The third station can process a unit in $7$ minutes.

is the bottleneck station, with a bottleneck time of

minutes per unit $($enter your response as a whole number$).$