Consider a small pharmacy where one pharmacist, one administrative assistant and one cashier work.

On average, $80\%$ of the customers come to fill prescriptions and $20\%$ purchase only over the counter $($OTC$)$ medication.

Customers who come in to fill a prescription drop off their script with the cashier and each brings only one script. This step takes on average $5$ minutes per customer.

The pharmacist fills all prescriptions and this step takes on average $15$ minutes per customer. After spending $15$ minutes on a script, the pharmacist realizes that $20\%$ of the scripts are problematic and cannot be filled as is$.$ Those scripts are routed to the administrative assistant for problem resolution.

The administrative assistant, on average, spends $40$ minutes resolving an issue on a script. Assume that all problems are eventually resolved. Those scripts are routed back to the pharmacist to be filled and this step takes $10$ minutes.

All customers pay the cashier. Check$-$out is on average $3$ minutes for an OTC customer and $6$ minutes for a customer who came in to fill a script.