A production line at V. J. Sugumaran's machine shop has three stations. The first station can process a unit in 8 minutes. The second station has three identical machines, each of which can process a unit in 21 minutes (each unit only needs to be processed on one of the three machines). The third station can process a unit in 10 minutes.

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