Two pipes A and B can fill a tank in 8 hrs and 12 hrs respectively. Another pipe C can empty it in 6 hrs. All the three are opened at a time. In what time will the tank be filled.

A can fill in 8 hrs, in 1 hour $=\frac{1}{8}$

B can fill in 12 hrs, in 1 hour $=\frac{1}{12}$

C can empty in 6 hrs, in 1 hour $=-\frac{1}{6}$

Time taken to completely fill =$\frac{1}{8}+\frac{1}{12}-\frac{1}{6}=\frac{3+2-4}{24}=\frac{1}{24}$

time taken to fill the tank is 24 hours