Bacteria Population Assume that the total number (in millions) of bacteria present in a culture at a certain time $t$ (in hours) is given by
$$N(t)=3 t(t-10)^{2}+40 \text { . }$$
a. Find $N^{\prime}(t)$ .
Find the rate at which the population of bacteria is changing at the following times.
b.8 hours $\qquad$ c. 11 hours
d. The answer in part b is negative, and the answer in part c is positive. What does this mean in terms of the population of bacteria?