Original circuit: ER = 8 volts ; IR = 8 mA
With 3 volt voltage source only: ER = 3 volts ; IR = 3 mA
With 5 volt voltage source only: ER = 5 volts ; IR = 5 mA
5 volts + 3 volts = 8 volts
5 mA + 3 mA = 8 mA
Notes:
This circuit is so simple, students should not even require the use of a calculator to determine the current figures. The point of it is, to get students to see the concept of superposition of voltages and currents.