Two students are playing a ball game. If they put 6 yellow ping pong balls in the bag, how many white ping pong balls should be put in if the game is fair.
两个同学玩摸球游戏，如果在袋子里放入6个黄色乒乓球，如果让游戏公平，应再放进去多少个白乒乓球．
<result>6</result>