A tank has four pipes P1, P2, P3, P4. The tank can be filled in 15 minutes by pipes P1, P2, P3 together. It can be filled in 20 minutes by pipes P1, P2, P4 together and it can be filled by pipes P1 and P3 together in 30 minutes. If all the pipes are opened together, then in how much time will the tank be filled?