An efficiency study conducted for Elektra Electronics showed that the number of Space Commander walkie-talkies assembled by the average worker during the morning shift t hr after starting work at 8 a.m. is given by N(t) = −t³ + 6t² + 15t (0 ≤ t ≤ 4).

(a) Find the rate at which the average worker will be assembling walkie-talkies t hr after starting work.