It takes 65,438+00 days for Master Zhang to complete work A alone, and 65,438+05 days to complete work B alone.

It takes Li Shifu 8 days to finish work A and 20 days to finish work B alone.

It takes 8 days for Li Shifu to finish A's work alone, and Zhang Shixian will finish B's work alone. After 8 days, the two will cooperate.

In eight days, Master Zhang made 8/ 15, leaving 7/15 ÷ (115+1/20) = 7/15.

Therefore, the fastest time is 8+4= 12 days.