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.?

(c)How many walkie-talkies will the average worker assemble between 9 a.m. and 11 a.m.?