so 1 coulomb = 6.245 x 10^18 electrons
meaning 4.0 x 10^18 electrons is only:
a) 0.64C
current is measured in amperes, which are coulombs per second. this means you take 0.64C/2seconds to get 0.32Amps
b) typically written as 320mA since we can divide by 1000. or keep it as 0.32A