In a series of oil-drop experiments, the charges measured on the oil drops were 3.20 × 10–19 C, –6.40 × 10–19 C, –9.60 × 10–19 C, and –1.12 × 10–18 C. What is the smallest difference in charge between any two drops? If this is assumed to be the charge on the electron, how many excess electrons are there on each drop?