Two pipes A and B can fill a tank in 18 and 24 hours respectively. If both the pipes are opened together, then at what time should pipe B be closed so that tank is completely filled in 12 hours?

8 h

Now A worked for 12 hours, so total work done by A = 12 × 4 = 48

Remaining = 72 - 48 = 24

B should be closed after = \(\frac{24}{3}\) = 8 hours