Suppose 20.0 kJ of heat is added to a system while at the same time the system does 5.0 kJ of work. By how much does the internal energy of the system change?
a) Increases by 10.0 kJ
b) Decreases by 5.0 kJ
c) Increases by 15.0 kJ
d) Decreases by 25.0 kJ
e) Increases by 20.0 kJ