Question

请问概率论1.25第二小题怎么做,答案是690/1421.

Answer 1

{(C10 2)/(C50 2)+(C18 2)/(C30 2)}/{(C10 1)/(C50 1)+(C18 1)/(C30 1)}

={90/(50*49)+18*17/(30*29)}/{1/5+3/5}

=(9/49*5+51/29*5)/(4/5)

=(9*29+51*49)/(19*29*5)/(4/5)

=2760/(19*29*4)

=690/(19*29)

=690/1421