Number of Topics to Study for on a Test (Brain Teaser)

I have a history midterm approaching and there is an identification section. It lists 24 different topics to write on. On test day, he will choose 12 of these topics and we choose 3 to write on. How many do I need to prepare for?

It makes me think of the following example: If you have 3 possible essays, 2 are chosen on test day, and you have to write on 1, then you only need to study for two of them. How does this apply to the original question?

Thanks so much! I will appreciate all the help I can get on this one. As of now, I will be studying all of the topics until I hear different.