James gets bored with his game so decides to play a different one.  That game promises 100 hours of gameplay but 80% of that is boring grinding.  However, the expansion does add another 30 hours of enjoyable gameplay.  How much enjoyable gameplay does James get?
The first game was 100*.8=<T>multiply(100, 0.8)=80.0 hours of grinding
So it was 100-80=<T>subtract(100, 80)=20 hours of good gameplay
The expansion brought the time of good gameplay to 20+30=<T>add(20, 30)=50 hours
