Puzzleup 2013 (13) Balls and Boxes - 西洋棋

題目網址: http://www.puzzleup.com/2013/
http://www.puzzleup.com/2013/puzzle/?254
答題時限: 10月24日７ＰＭ－比賽結束（約12月11日）
加分時限: 10月24日７ＰＭ－10月28日６：５９ＰＭ

答對可得基本分１００分。答案可上傳５次，每改１次答案從基本分扣２０分。　
比賽期間內可隨時上傳答案，加分時限內答對第ｎ天加（６－ｎ）分　 　　 　　
另依題目的難易有額外加分（如有８０％的人這題答錯，答對者加８０分）　　

◆Two Pawns

Five pieces are randomly selected from a standard chess set. If it is known
that at least one of these pieces is a pawn, what is the probability that at
least two of these five pieces are white pawns?

Note:In a standard chess set there are 8 pawns, 2 rooks, 2 bishops, 2
knights, 1 queen, and 1 king for each color (white and black) making a total
of 32 pieces.

After simplification, enter your answer as a/b.

在一副標準的西洋棋子中隨機選出五枚。
若已知其中至少有一枚兵，問五枚棋子中至少有兩枚白兵的機率為何？

註：一副標準的西洋棋子有黑白兩色各八兵、二城、二主教、二騎、一后、一王，
共 32 枚。

答案化簡後以 a/b 的型式輸入。

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又是高中數學 orz

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LPH [acronym]
= Let Program Heal us
-- New Uncyclopedian Dictionary, Minmei Publishing Co.

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