A production line at V. J.? Sugumaran's machine shop has three stations. The first station can process a unit in 5 minutes. The second station has three identical? machines, each of which can process a unit in 18 minutes? (each unit only needs to be processed on one of the three machines). The third station can process a unit in 7 minutes.

(Station 1,2, or 3) is the bottleneck? station, with a bottleneck time of (.....?) minutes per unit ?(enter your response as a whole? number).