In a hypothetical universe, an oil-drop experiment gave the following measurements of charges on oil drops: –5.55 × 10–19 C, –9.25 × 10–19 C, –1.11 × 10–18 C, and –1.48 × 10–18 C. Assume that the smallest difference in charge equals the unit of negative charge in this universe. What is the value of this unit of charge? How many units of excess negative charge are there on each oil drop?