A motor with an efficiency of 75 percent must supply 240 j of useful work. What amount of work must be supplied to the motor.

E = 75% = 0.75, Work Output = Useful Work = 240 J, Work Input = ?

Efficiency, E = Work Output / Work Input

0.75 = 240 / Work Input

Work Input = 240 / 0.75

Work Input = 320

Amount of work supplied = Work Input = 320 J

I hope this explains it.