You did:
5 episodes / 4 hours = 1.25 episodes/hour
You found how many episodes they watched in one hour, when you should have found how many hours they watched one episode. This is because you need to find how long it took them to watch 24 episodes.
So instead:
4 hours / 5 episodes = 0.8 hours/episode
Then you multiply 0.8 and 24 = 19.2 hours
