Hey! Nothing is wrong with MIF! As long as you understand what is needed, then it's all good! :)
By the way, once you have exhausted the hardest questions or seem to have demolished the book, it is recommended that you seek other textbooks that have questions to challenge you. That is, to give...
Probability is logic. To see quickly improvments, I recommend that you use logic and see why the solution is that way
Example working out and how to go about analyzing the sol'n or your sol'n,
1. The question
2. Step A (why?)
3. Step B (why?)
4. Step C (why?)
5. Answer (why?)
If you can...