Answer: 0.125mm... I don't know why the guy before me was rated so low I got the exact same answer.
Question: The expression 0.01E+0.003E^2 gives the thickness in millimeters (mm) of the insulation needed for a high-voltage cable. E is the number of kilovolts carried by the cable.what thickness of insulation material is needed for a 5 kilovolt cable?