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标题: gwd7-14 [打印本页]

作者: jojofgasw 时间: 2004-12-12 13:41 标题: gwd7-14

Q14:

Six cards numbered from 1 to 6 are placed in an empty bowl. First one card is drawn and then put back into the bowl; then a second card is drawn. If the cards are drawn at random and if the sum of the numbers on the cards is 8, what is the probability that one of the two cards drawn is numbered 5 ?

我做出来是1/3，答案是2/5，请帮帮忙！

我觉得加起来等于八，只有2和6，3和5，4和4三种组合，所以是1/3！

作者: mary1 时间: 2004-12-12 13:47

The number of ways that the sum of the numbers on the 2 cards is 8 is 5，i.e., 2,6 6,2 3,5 5,3 4,4. So th probability that one of the 2 cards drawn is numbered 5 is obviously 2/5.

作者: smart 时间: 2004-12-13 03:27

我觉得加起来等于八，只有2和6，3和5，4和4三种组合，所以是1/3

在做这类题时，2和6，6和2是两种情况。
我记得以前有个题好像是问生双胞胎生一男一女的概率。
开始我就理解成两男，两女，一男一女，所以概率为1/3.
其实应该是两男，两女，一男一女，一女一男，所以概率为1/4。

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