1/甲+1/乙=1/20
1/乙+1/丙=1/30
1/丙+1/丁=1/40
三式相加得
1/甲+2*(1/乙+1/丙)+1/丁=13/120
则1/甲+1/丁=13/120-2/30=5/120=1/24
甲丁合作需:1/(1/24)=24天完成