Below is a figure of a production line. Rectangles $($A$,$B$,$C$)$ denote machines where processing

of jobs takes place, and triangles $($P$,$Q$)$ are buffers for storing items that may be waiting

for the next machine to be available. Process$-$B involves a chemical treatment where up

to $500$ items can be treated simultaneously as a group, and the treatment takes five hours

regardless of the number of items. Process$-$A operates on each item one$-$at$-$a$-$time taking

$45$ seconds per item. Similarly, Process$-$C takes $40$ seconds per item.