This is the first book of Math Contest Books Series. The book introduces a powerful problem solving technique - the mass points method. The book can be used by students preparing for math competitions such as Mathcounts, AMC 10/12/AIME.
Second book of Math Contest Books Series:
https: //www.amazon.com/Balls-Boxes-Yongcheng-Chen/dp/1540390578
Third book of Math Contest Books Series: The Area Method
https: //www.amazon.com/dp/1540856410