I just finished my second year as a mathematics student at university. At university, we learn about advanced mathematics and problems. However, I'm also interested in some problems that doesn't acquire deep results from mathematics or a certain theory to solve them.
Now I'm looking for a good guide to learn more about this type of problem-solving. I want some pdf or a book that gives some brief introduction to a problem solving technique and after that a large bunch of problems. I don't want these problems to be too easy but I also don't want them to be too hard so that I have to know much modern results in mathematics to understand them.
Does anyone of you knows such a guide at my level (university student)?