急，求答案，谢了
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1/2*1/3+1/3*1/4+1/4*1/5
=1/3*(1/2+1/4)+1/4*1/5
=1/3*3/4+1/4*1/5
=1/4+1/4*1/5
=1/4*(1+1/5)
=1/4*6/5
=3/10
=1/3*(1/2+1/4)+1/4*1/5
=1/3*3/4+1/4*1/5
=1/4+1/4*1/5
=1/4*(1+1/5)
=1/4*6/5
=3/10
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