A patient receives a 150 -mg injection of a drug every 4 hours. The graph shows the amount $f(t)$ of the drug in the bloodstream after t hours. Find
$$\lim _{t \rightarrow 12^{-}} f(t) \quad \text { and } \quad \lim _{t \rightarrow 12^{+}} f(t)$$
and explain the significance of these one-sided limits.