On a weekday morning in a neighborhood coffee shop, on an average, $90$ customers are served every hour. Each customer waits on an average for $4$ minutes, from entering the shop till getting the order prepared. Finally, on average, $6$ customers can be seen waiting, either in line to place the order, or in front of the barista.

According to Little's Law, the number of customers waiting in the coffee shop, either in line or in front of the barista, can be treated as

Flow Rate

the ratio of Flow Rate and Flow Time

Inventory

Flow Time