You can calculate the volume from the buoyancy. Then probably the assumption is made that the volume of the alloy is composed of the mixing ratio of the two parts, is a simplification because the alloy will have a greater density.
Volume(alloy)=Volume(Au)+Volume(Ag).
So set up the mixing...
In such tasks, many things are simplified, because otherwise it becomes too difficult, one then also no longer sees the principle represented. So there is no friction, which is why the horizontal displacement can be done with zero force. And if force F=0, F*s is also 0.
Perhaps MgL/18 was not understood.
My explanation for this is:
given:
L length of the chain
1 third hangs down
M Mass of the chain
searched:
Work for stroke of chain on table
Formula:
W=mgh
The thickness of the chain is neglected, otherwise thickness/2 would have to be added to the lifting...