I'm nearly done studying for the GRE, this is a huge gap that needs to be filled. None of the test prep companies have the software know-how to build anything fun. I think your product can succeed. My 2 cents (take with a grain of salt) is that real learning is difficult, so your ideal learning algorithm will constantly be giving people problems they're bad at, and attacking their weaknesses. This could discourage people who are used to 'studying' as just doing busywork/rote problems. Either provide education on this piece, or mix in some easy stuff. Good luck I love your idea.