Question

What is the input of energy for a device that provides 50 units of useful energy and has an efficiency of 0.55?

Answer 1

If that's the case, then  

                          50 units = 0.55 x the input energy

Divide each side by  0.55 :        

                   50 units/0.55  =  the input energy = 

                               90 and 10/11 units