As can be seen from the figure: Before 15 minutes, the pipe A was single-opened, so it can be calculated that the pipe A can inject 1.200 cubic centimeters of water per minute, and the total amount of water injected is 1.8000 cubic centimeters. At 15, the two pipes are opened at the same time, and 8 1000 cubic centimeters of water is injected at10. 81000-18000 = 63000 cubic centimeters (this is injected by water pipes a and b). 63,000/10 = 6,300 cubic centimeters (water injected into pipes A and B every minute). The volume of the water tank is 60*30*45=8 1000 cubic centimeter, and 8 1000/6300= 12.86 (min).