b$)$ The process time of the overall system is determined by the bottleneck. Bottleneck is found in a$),$ which is the welding operation, and it is $2$ units$/$hr$=0\mathrm{.}5$ hr$/$unit$.$

c$)$ Sawing and Sanding are not operating simultaneously. Sawing comes before Sanding. So$,$ the process cycle time is wrong because the answer combines sawing and sanding for no reason.