There was 1/6 water, and the remaining1-(1/6) = 5/6 = 50/60.

After seven cycles, there is (50/60)-7*(7/60)= 1/60.

Then, when A starts water injection, the required time is (1/60) ÷ (1/3) =1/20 hours.

Therefore, the total time required for * * * is 4×7+( 1/20)=28.05 hours.