books.google.com - This book focuses on combinatorial problems in mathematical competitions. It provides basic knowledge on how to solve combinatorial problems in mathematical competitions, and also introduces important solutions to combinatorial problems and some typical problems with often-used solutions. Some enlightening...http://books.google.com/books/about/Combinatorial_Problems_in_Mathematical_C.html?id=c2538EUnyggC&q=triples&utm_source=gb-gplus-shareCombinatorial Problems in Mathematical Competitions