A machine has an efficiency of 91 percent if the enery supplied to the machine is 4000 watts what is the amount being used for work?

Turn the percentage into a decimal by dividing by 100: 91% --> .91

Work out = (efficiency) * (work in)

dividing by a unit of time gives =>

Power out = (efficiency) * (power in)

P_o = .91 * 4000 Watts = 3640 Watts