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 so that Y&Y can 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, one person cooks a batch of orders in a wok. The cooking depends upon the number of orders in the batch. The time to cook just one order is 3 minutes (including setup time). For each additional order in the batch, it takes 0.5 minutes more to cook. Thus, cooking two orders in a batch takes 3.5 minutes, cooking three orders takes 4 minutes, and so on.

The other process is bagging and accepting payments (done by a separate person), which takes 0.80 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 places.) Please include the explanation.