Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 75: 47

$37$

We know that one negative charge is i.e charge on electron $1.6\times 10^{-19}C$ Given that the total charge on oil drop is $5.93\times 10^{-18}C$ Thus the number of negative charged on the oil drop$=\frac{5.93\times 10^{-18}C}{1.6\times 10^{-19}}=37$