设每个的的工作效率分别为1/a,1/b,1/c,1/d,1/e,则
1/a+1/b+1/c=1/7.5=2/15.(1)
1/a+1/c+1/e=1/5.(2)
1/a+1/c+1/d=1/6.(3)
1/b+1/d+1/e=1/4.(4)
(1)+(2)+(3)+2*(4)得：
3(1/a+1/b+1/c+1/d+1/e)=3/4
1/a+1/b+1/c+1/d+1/e=1/4
5个合作共4时完工