Recently, I felt lack of knowledge and techniques for solving various number of cases, probability and expected number problems.

I did not managed to solve a lot of problems including

• Dstorv,
• Gem Island,
• Do I wanna know,
• Solitaire, and
• BBQ Hard.

(For the first three problems, I did not managed to solve problem in contest. And, I think it led to unsatisfying results.)

I really want to be good at this kind of problems, so could anyone give sufficient number of problems, or materials (including book) to improve my skill?