王夫之在《读＜通鉴＞论》中因“魏诏守令劝课农事”而发议论说：“历来传为美谈、德政的所谓‘劝农’，实质上等于伤农．”有人把这称之为“王夫之定律”．据此分析“王夫之定律”（  ）

    Rick is a good man．He makes birthday cakes in a shop．His cakes are great and have very good prices，so many people buy his cakes．He makes friends with lots of his customers(顾客)．They all like him very much．One evening．he wants to go home，but many of his old customers come in．Rick smiles(微笑)to them，“I am sorry．It is late today．If you need cakes，please come tomorrow．”One of them takes out a nice cake and says to him，“You make so many great cakes for us．This birthday cake is for you．Happy birthday, Rick．”

一个布袋中装有只有颜色不同的a（a＞12）个小球，分别是2个白球、4个黑球，6个红球和b个黄球，从中任意摸出一个球，记下颜色后放回，经过多次重复实验，把摸出白球，黑球，红球的概率绘制成统计图（未绘制完整）．根据题中给出的信息，布袋中黄球的个数为　 ____

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