A5,400 w motor is used to do work. if the motor is used for 640 s, about how much work could it do?

Since P=E/t,
where P=power (W),
E=energy (J) and
t=time (s).

Just rearrange this formula to obtain E=P*t=5400*640=3456000 J.

Energy and work are very related since energy is defined like the ability of doing work. Actually work is also measured in J so the answer is the one above.
Work = 3456000 J

B, if it was a lower watt she could put it in for her normal time and not burn it