设甲乙丙丁的效率分别为X、Y、Z、M
则X+Y+Z=1/18(1)
Y+Z+M=1/15(2)
X+Y+M=1/12(3)
X+Z+M=1/20(4)
(1)+(2)+(3)+(4)得,3(X+Y+Z+M)=1/18+1/15+1/12+1/20=92/360
所以X+Y+Z+M=92/1080(5)
(5)-(2)得,X=92/1080-1/15=20/1080=1/54
所以甲队单独做用1÷(1/54)=54天
设甲乙丙丁的效率分别为X、Y、Z、M
则X+Y+Z=1/18(1)
Y+Z+M=1/15(2)
X+Y+M=1/12(3)
X+Z+M=1/20(4)
(1)+(2)+(3)+(4)得,3(X+Y+Z+M)=1/18+1/15+1/12+1/20=92/360
所以X+Y+Z+M=92/1080(5)
(5)-(2)得,X=92/1080-1/15=20/1080=1/54
所以甲队单独做用1÷(1/54)=54天