**Two pipes A and B can fill a tank in 12 hours and 18 hours respectively. How long will they take to fill the tank together?**

**Three pipes can fill a tank in 5 hours, 10 hours, and 15 hours respectively. How long will they take to fill the tank together?**

**Two pipes A and B can fill a tank in 8 hours and 16 hours, respectively. However, there is a third pipe C that can drain the tank in 12 hours. If all three pipes are opened together, how long will it take to fill the tank?**

**Three pipes A, B, and C can fill a tank in 30 minutes, 20 minutes, and 10 minutes respectively. When the tank is empty, all three pipes are opened. A, B, and C discharge chemical solutions P, Q, and R respectively. What is the proportion of solution R in the liquid in the tank after 3 minutes?**

**Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in:**