This is a chemistry question that i don't know how to do. need some

In the millikan oil drop experiment they determined that every drop had a charge which was a while number multiple of -1.60x10^-19. if a drop has a total charge of -9.60x10^-19 then how many excess electrons are contained within the drop?

Chemistry worksheet - i am not sure what they are asking for exactly?

How do you know when a chemical reaction has occurred