To solve this problem, we need to determine the time it takes for pipe A alone to fill the tank. We can do this by finding the reciprocal of the time it takes for pipe A to fill the tank. The reciprocal of 12 minutes is 1/12, which means that pipe A can fill 1/12th of the tank in 1 minute.

Next, we need to find the time it takes for both pipes A and B to fill the tank together. We can find this by taking the reciprocal of the sum of the reciprocals of the individual times. The reciprocal of 12 minutes is 1/12 and the reciprocal of 16 minutes is 1/16. So, the reciprocal of the sum of these values is 1/12 + 1/16 = 7/48. This means that both pipes A and B together can fill 7/48th of the tank in 1 minute.

Now, let`s say that pipe B is closed after t minutes. In those t minutes, pipe A and B together can fill t * (7/48) of the tank. We want this amount to be equal to the remaining part of the tank which is (1 - t/9). So, we can