甲工效:1/4=1/4
乙工效:1/5=1/5
丙工效:1/6=1/6
丁工效:17=1/7
(1-1/6)/[(1/4+1/6)-(1/5+1/7)]
=5/6/[5/12-12/35]
=(5/6)/(31/420)
=350/31
∵