Consider a system with N users, a CPU and two disks. A request from the users first

visits the CPU. After processing at the CPU, with probability $0\mathrm{.}8$ the request visits disk

A$,0\mathrm{.}16$ it visits disk B and $0\mathrm{.}04$ it is completed and returns to the user. After processing

at either disk, the request returns to the CPU. The mean think time at the users is $5$

seconds, the mean processing times $($per visit$)$ at the CPU, disk A$,$ and disk B are $30,25$

and $40$ milliseconds, respectively.

$($a$)$ What is the bottleneck device?