As the days go by, you want to understand how the number of customers is changing over time. You start to calculate the derivative of $$f(t)$$, which represents the rate of change of the number of customers. You find that $$f'(t) = 10$$ for $$t$$ close to 12:00 PM. This means that for every minute that passes, the number of customers increases by 10.
Only briefly in projects. Full metric space theory is in Rudin or Munkres. Abbott stays in ℝ (the real numbers), which is ideal for a first course.
