Loking for C# practice Problems

Hi, I am looking for C# interview style programming problems for my self to practice with. I would like to find some material that tests problem solving skills, efficiency of implementation as well as knowledge of the language, though not overly complicated. Something along the lines of algorithm implementation like fibonacci series, problems that ought not to take more that say 20 mins max to solve. I'd appreciate your comments, suggestions, links, resources, ect. P.S. Can anyone recommend C# equivalent books to "Effective C++" & Efficient C++"?