Two pipes A and B can individually empty a tank in 18 minutes and 36 minutes respectively. Pipe A is shifted to half the height of the tank (It can only empty till half the tank). What is the time taken to empty the tank if both the pipes are opened together? (in minutes)

Pipe A's rate = 1/18 per minute. Pipe B's rate = 1/36 per minute. Since Pipe A can only empty half the tank, it will empty 1/2 in 9 minutes. During these 9 minutes, Pipe B empties 9/36 = 1/4 of tank. Remaining = 1 - 1/2 - 1/4 = 1/4. Now Pipe A stops. Pipe B alone empties remaining 1/4 in (1/4) / (1/36) = 9 minutes. Total time = 9 + 9 + 6 = 24 minutes.