after _______ minutes time B should be closed so that the tank is full in 18 minutes
Interpretation 1: After x minutes should B be closed, so that A takes 18 minutes to fill the rest of the tank.
ie, it takes 18 minutes AFTER closing B
Interpretation 2: After x minutes should B be closed, so that A fills the rest of the tank, and the total time taken is 18 minutes.
ie, it takes 18 minutes in TOTAL.
Interpretation 1 gives you the answer 3.43 minutes.
Interpretation 2 gives you the answer 8 minutes.