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? Let's be accurate as possible.
The first game was 100 * .8 = 80 hours of grinding. So it was 100 - 80 = 20 hours of good gameplay. The expansion brought the time of good gameplay to 20 + 30 = 50 hours.
The answer: 50.