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 $6\mathrm{.}5$ minutes. For each additional order in the batch, it takes $0\mathrm{.}5$ minutes more to cook. Thus, cooking two orders in a batch takes $7$ minutes, cooking three orders takes $7\mathrm{.}5$ minutes, and so on$.$