條件機率問題求解釋

(B) A farmer decides to test four fertilizers for his soybean fields. He buys 32 bags of fertilizers, eight bags from each kind, and tries them randomly on 32 plots, eight plots from each of fields A, B, C, and D, one bag per plot. If from type I fertilizer one bag is tried on field A and three on field B, what is the probability that two bags of type I fertilizer are tried on field D?
我的答案是:
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請問這個答案正確嗎?
如不正確可否講解一下?
謝謝!!!

更新1:

HI~ 謝謝'自由自在"的解答,但還是有些不明白題目不是說其中一種化肥（共有８小包）有１小包在Ａ田，３小包在Ｂ田，問此種化肥有２小包落在Ｄ田的機率嗎？還是說我題目理解錯了？（英文程度不好一個字一個翻譯的）用掉４包，全部化肥剩下２８包，其中有８包是落在Ｄ田的，此種化肥剩下４包，有２小包落在Ｄ田，請問這樣算法有錯嗎？希望能講解一下！謝謝！

回答 (1)