C5里取3,得(3!*(5-3)!)/5!= (2*1)*(3*2*1)/5*4*3*2*1=1/10
C4里取2,得(2!*(4-2)!)/4!= (2*1)*(2*1)/4*3*2*1=1/6
C5里取1,得(1!*(5-1)!)/5!= 1*(4*3*2*1)/5*4*3*2*1=1/5
则原题可化为,1/10+1/6*1/5=2/15
C5里取3,得(3!*(5-3)!)/5!= (2*1)*(3*2*1)/5*4*3*2*1=1/10
C4里取2,得(2!*(4-2)!)/4!= (2*1)*(2*1)/4*3*2*1=1/6
C5里取1,得(1!*(5-1)!)/5!= 1*(4*3*2*1)/5*4*3*2*1=1/5
则原题可化为,1/10+1/6*1/5=2/15