feifei section 4-15

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feifei 4-15

15. There are six cards each with a number from 1 to 6. Draw one card randomly and then put it back into a bowl. If one card is to be drawn randomly, and the sum of two cards is 8, what’s the probability that the card is 5?

 

 

【答案】2/5

【思路】因放回，两张cards之和＝8有5种可能，2 6，6 2，3 5，5 3，4 4，其中出现5的可能有2种。

 

但是这个不考虑对整个取样的么，整个取样的possibility 是p6 2, 然后里面有两种情况包括5 （5+3， 3+5）, 为什么答案只拿5种相加得8 的情况而不是整个取样情况做分母呢？

谢谢~