一项工程，一二三队合干需12天一三五队合干需7天二四五合干需8天一三四合干需42天问一二三四五队合做需几

设一二三四五队单独干分别需要a,b,c,d,e天
根据题意得:
1/a+1/b+1/c=1/12……1
1/a+1/c+1/e=1/7……2
1/b+1/d+1/e=1/8……3
1/a+1/c+1/d=1/42……4
1+2+2*3+4可以得到
3(1/a+1/b+1/c+1/d+1/e)=1/2
(1/a+1/b+1/c+1/d+1/e)=1/6
1/(1/a+1/b+1/c+1/d+1/e)=6
所以一二三四五队合做需6天