16.随机从1-20里抽取5个不重复的数，问抽到5个数中其中两个是10和20的概率是多少（1/19

497460618 · 发表于 2012-1-29 23:30:52

sdcar2010 · 发表于 2012-1-29 23:43:26

Total possibilities for picking out 5 different numbers from 1-20:

(20*19*18*17*16)/5! =19*3*17*16

If two of those five are 10, and 20, the total possibilities are:

(18*17*16)/(3!) = 3*17*16

Then one over the other = 1/19

497460618 · 发表于 2012-1-29 23:58:21

sdcar2010 · 发表于 2012-1-30 00:00:26

Do you know the relationship of possibilities and probability (chance)?

497460618 · 发表于 2012-1-30 00:03:27

I can not understand (18*17*16)/(3!) = 3*17*16

sdcar2010 · 发表于 2012-1-30 00:12:57

18/6 = 3

Since 2 numbers were picked already, you only need to pick three more numbers from the remaining 18 candidates.

zhualei1982 · 发表于 2012-2-4 12:51:54

谢谢

qxec586 · 发表于 2012-2-4 20:59:43

那个我不会打符号算式就是1-(C5 19/C5 20)

qxec586 · 发表于 2012-2-4 21:00:52

不是是1-（C5 18/C5 19）

kevin8951 · 发表于 2012-2-5 14:31:14

分母是C（20，5）意思是20个里面取5个。分子是C（2,2）*C（18,3）意思是10和20这2个肯定要取，然后从剩下18个里面取3个。算出来正好是1/19

16.随机从1-20里抽取5个不重复的数，问抽到5个数中其中两个是10和20的概率是多少（1/19

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