Discrete math isn't arithmetic. When you check the solution, don't just see if "42" is the final number. Check if your assumptions matched the author's. Did you use the right formula for permutations vs. combinations?
This book single-handedly saved my grade in Computer Science theory. It is ugly. It is dense. The cover is boring. But it works. 2000 Solved Problems In Discrete Mathematics Pdf
If you are a Computer Science or Engineering student, you have probably heard the dreaded whisper in the library: "Discrete Math is a weeder course." Discrete math isn't arithmetic