Electrons have a charge of q $=-1\mathrm{.}602\backslash$times $10-19$ C$.$ A student observes that a Styrofoam cup has a total charge of Q $=-0\mathrm{.}25\backslash$mu C$.$ a$.$ Write an expression for the number of excess electrons N in the cup. b$.$How many excess electrons are in the cup?