回复：依次任取7个小于等于10的正整数，求其和为20的概率？

Hi,Brother,
If you want to know the exact answer you have to use computer.Twft is right,it is from generating function.
007's answer is reasonable.I do it by subnormal distribution.Mean=5.5*7=38.5.Sigma=(38.5-7)/4=7.875,z1=19.5/7.875=-2.476,z2=(20.5-38.6)/7.875=-2.286.P(E)=T(2.476)-T(2.286)=.9933-.9888=.0045.