Oh, well, one quarter is equal to 1/4, the cubic shaped fish tank is 40 inches tall, and the piece of metal is 30 inches in height, that'll leave 10 inches left of space at the top of the tank, (40 - 30 = 10), and the water level will become 4 quarters full, since the metal took up most of the space and one quarter of water is larger than 10 inches. So you're answer is 4 quarters full. ;-)
I hope I helped! =D
