Worker Efficiency 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.
(b) At what rate will the average worker be assembling walkie-talkies, per hour, at 9 a. m.?walkie-talkies/hour At what rate will the average worker be assembling walkie-talkies, per hour, at 10 a. m.?
(c) How many walkie-talkies will the average worker assemble between 9 a. m. and 10 a. m.?walkie-talkies