The Yum and Yee food truck near the business school serves customers during lunch hour by taking orders and making fresh batches of stir fry. Customers have only one choice during the lunch hour, since the objective is to maximize the number of customers served. Assume that each customer places just one lunch order, and all lunch orders are the same size-one unit of stir fry.

The stir fry cooking works in this manner. First, a batch of orders is cooked in a wok by one person. The cooking depends upon the number of orders in the batch. The time to cook just one order is 4.5 minutes. For each additional order in the batch, it takes 0.5 minutes more to cook. Thus, cooking 2 orders in a batch takes 5 minutes, cooking 3 orders takes 5.5 minutes, and so on.

The other process is bagging and accepting payments (done by a separate person), which takes 0.90 minutes per order.

If Yum and Yee operates with batch sizes of 4 units, what is their process capacity (in orders per minute)? ( The accuracy should be of two digits after the decimal place).