As part of a quality-control program, the chess sets manufactured by Jones Brothers are subjected to a final inspection before packing. The rate of increase in the number of sets checked per hour by an inspector $t$ hr into the 8 A.M. to 12 noon shift is approximately
$$N^{\prime}(t)=-3 t^{2}+12 t+45 \quad(0 \leq t \leq 4)$$
a. Find an expression $N(t)$ that approximates the number of sets inspected at the end of $t$ hours.
b. How many sets does the average inspector check during a morning shift?