In a capacity$-$constrained process, why are we interested in locating the bottleneck?

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In a mail$-$order company there are three steps to complete when a call comes in$.$ First the order is received and typed into the system by employee $1.$ Then employer $2$ checks the stock to see if the product is available. Finally, employee $3$ arranges the shipment from that location. The processing times are:

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Receive Order: $5$ minutes$/$order $($P $=5$ minutes$/$unit$)$

Check Stock: $2$ minutes$/$order $($P $=2$ minutes$/$unit$)$

Arrange Shipment: $10$ minutes$/$order $($P $=10$ minutes$/$unit$)$

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$1$a$.$ What is the flow unit?

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$1$b$.$ What is the capacity of the Receive Order station in orders per hour?

Hint: with one worker at a resource, the Resource Capacity $=1/$p$,$ and there are $60$ minutes$/$hour$.$