Two pipes A and B can fill a tank in 18 minutes and 24 minutes respectively. If both the pipes are opened simultaneously, then after how much time should pipe B be closed so that the tank is full in 12 minutes?

8 minutes

Let B be opened for X min,

According to the question,

\(\frac{12}{18}\) + \(\frac{X}{24}\) = 1,

\(\frac{X}{24}\) = 1 - \(\frac{2}{3}\) = \(\frac{1}{3}\)

X = 8 min.

Therefore, Pipe B is opened for 8 mins.