Larry installs a string of battery powered Christmas lights on his truck. The string of lights has the following charactersistics: lenth= 21 m, mobile charge density = 8.5e28 electrons/m^3, cross-sectional area=1.8e-7 m. when the lights are turned on, the current is .4 A. How long does it take an electron to go 21 m from one end of the light string to the other end?