It is F that will fill up first , if you look closely at the details you will notice that D and H are closed , so water cannot pass through them , and that will change the direction of the water to fill up the tank F !
It is F that will fill up first , if you look closely at the details you will notice that D and H are closed , so water cannot pass through them , and that will change the direction of the water to fill up the tank F !