Electrical energy is used to run the fan
Here as per given condition 750 J of electrical energy is used to run the fan which is converted into Kinetic energy as 400 J
So here we can see that 350 J of energy is lost against many other type of frictional and resistive loses.
So here we can say that out of 750 J of energy only 400 J is used to run the fan and rest amount of energy is lost against friction.
also we can say that efficiency of this fan will be
[tex]\eta = \frac{Useful}{Total}[/tex]
[tex]\eta = \frac{400}{750}[/tex]
[tex]\eta = \frac{8}{25}[/tex]