Example $8$: Little$$s Law $($Relating the average number of units in a process to the average flow rate and the average flow time$)$

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$$Suppose that the average time a gear case $($used for an outboard marine motor$)$ spends at a machine center is three hours.

An average of four gear cases arrive at the machine center per hour.

What is the average number of gear cases waiting and being processed $($work in process$)$ at the machine center?

$$ Work in process $($WIP$)$ consists of items, such as components or assemblies, needed to produce a final product in manufacturing.