(1-1/15×2-1/12×2)÷1/12
=(1-2/15-1/6)÷1/12
=7/10÷1/12
=8.4(天)
答:还要8.4天完成.
(1—1/8×2)÷15=1/20
1/8-1/20=3/40
答:乙每天完成这项工程的3/40.