In Millikan's oil drop experiment, the charge on an oil drop is calculated to be $6.35\times {10}^{-19} \mathrm{C}$. The number of excess electrons on the drop is

The nuclear radius of a certain nucleus is $7.2 \mathrm{fm}$ and it has a charge of $1.28\times {10}^{-17} \mathrm{C}$. The number of neutrons inside the nucleus is,

$($charge on one proton$=1.6\times {10}^{-19} \mathrm{C})$