Payment Program Queue

Your question is short on detail but here's a possible approach. I'm assuming that there is just a single queue with customers going to the first empty checkout rather than separate queues for each cashier:

1. Create a Customer class with a single 'MinutesToServe' property recording how many minutes it will need to get served at the checkout. For simplicity, you could assume that the payment to be made by that customer is proportional to the time it takes to get served.

2. Create a Queue and add some initial Customer objects to it.

3. Create a Cashier class with a Customer property, namely the Customer currently being served (null if no-one is being served), and a TotalPayments property containing the total amount received so far from customers served.

4. Create an array of Cashier objects from between 2 to 10.

5. Every minute the Timer needs to do the following:

(a) For each Cashier, decrement the current customer's MinutesToServe property. 

(b) When a Customer's MinutesToServe property reaches zero, then increment the Cashier's TotalPayments property by the appropriate amount and take the next Customer from the queue.

6. Every 'x' minutes, the Timer needs to add a random number of new Customers to the queue and set their MinutesToServe property to suitable random numbers.