c$.$ ANSWER ANYONE:

$)=(10$ marks

i$.$ Athens Technical Service $($ATS$),$ USA is pioneer in producing printed circuit boards $($PCBs$).$ In that one of the manufacturing processes is solder masking, which is performed by a automatic mask$-$dispensing machine. It keeps solder out of the areas where hardware is attached. The thickness of the solder mask is critical here. If the materials is added too much $($thick$),$ it may lead to crack or seep through the board material into holes that may lead to rejection of PCB board. Thin or uneven application may allow solder to fill the open areas. The nominal value of the solder thickness is $2$ mm $,$ and the loss caused by deviation from the nominal value is Rs$.120.$ The current control limit $({}_0)$ is $1\mathrm{.}5$ mm $,$ and the average maintenance interval $f$ the mask$-$dispensing machine is once every $5$ days $(120$ hours$).$ The solder thickness is checked once every $5$ hours at a cost of Rs$.10,$ and the average preventive maintenance cost to adjust the machine is Rs$.50.$ Time lag is not considered.

$($a$)$ What is the optimal checking interval?

$($b$)$ What is the preventive maintenance limit$?$

$(3)$

$($c$)$ What is the loss per unit time?

$(3)$

$($d$)$ If you are going to design a different masking system, what type of parameters would you consider?

$(1)$