It is known that $45 \%$ of all aluminum cans distributed will be recycled each year. A beverage company uses 250,000 lb of aluminum cans. After recycling, the amount of aluminum, in pounds, still in use after $t$ years is given by
$$N(t)=250,000(0.45)^{t}$$
(Source: The Container Recycling Institute.)
a) Find $N^{\prime}(t)$
b) Interpret the meaning of $N^{\prime}(t)$