9、A, B, C, D and Xiaoqiang, five classmates, are competing in a chess tournament, where every two of them play one match. So far, A has played 4 matches, B has played 3 matches, C has played 2 matches, and D has played 1 match. How many matches has Xiaoqiang played?
9.【Solution】 "Jia has played 4 games", which means Jia has played 1 game each with Yi, Bing, Ding, and Xiaoqiang (Xiaoqiang has played 1 game with Jia).
"Ding has played 1 game", which confirms that Ding has only played with Jia.
"Yi has played 3 games", which means Yi has played 1 game each with Jia, Bing, and Xiaoqiang (Xiaoqiang has played 1 game with Yi).
We now know that the 2 games Bing has played are 1 game each with Jia and Yi,
Therefore, Xiaoqiang has played 2 games.