First calculate the volume of the aquarium by multiplying its length, width and height: 4 ft * 6 ft * 3 ft = <<4*6*3=72>>72 cubic ft
Then figure out what proportion of the aquarium is full after the cat knocks it over: 1/2 * 1/2 = 1/4
Then figure out what proportion of the aquarium is full after Nancy refills it: 3 * 1/4 = 3/4
Now multiply the proportion of the aquarium that's full by the aquarium's volume to find out how much water is in it: 72 cubic ft * 3/4 = <<72*3/4=54>>54 cubic ft
#### 54