The rate at which a machine operator's efficiency, E (expressed as a percentage), changes with respect to time t is dE given by dt - =30- 8t, where t is the number of hours the operator has been at work. a) Find E(t), given that the operator's efficiency after working 2 hr is 76%; that is, E(2) = 76. b) Use the answer in part (a) to find the operator's efficiency after 3 hr; after 5 hr