丙的工作效率=[1-4×(1/24+1/30)]÷7
=(1-4×3/40)÷7
=(1-3/10)÷7
=7/10÷7
=1/10
则由甲乙丙三队同时开工修这条公路,1÷(1/24+1/30+1/10)
=1÷7/40
=40/7天完成