To grow past certain output rates, we must add another technologist. The marginal cost is high when we have to add a technologist. It's low otherwise. Once we grow past 30 per day, we can expand by more tens to 40 or 50 per day without having to add more technologists. Here's a small slice of that table again, for your reference.
Marginal cost is the concept to use when considering changes.
Sound applet You compare the costs with the change to the cost without the change. The difference is the marginal cost of the change. Compare that with the marginal benefit of the change to decide whether the change is advantageous. Q, divided by Q. Average cost is sometimes mistakenly used in place of marginal cost. Revenue minus cost.