The efficiency of a machine is 60%. If the work done on the machine is 660 J, then what is the work done by the machine

Efficiency is simply, the ratio of the output to the input of any system .That is, the amount of energy produced that is converted into work.So we can also say,

Now, as Efficiency of the machine= 60%
P _out= 60% of P_in
= 0.6 X 660 J
=396J