Answer:
192 fishes
Explanation:
The tank is a rectangular prism, so its volume can be calculated multiplying its length, width and height:
Volume = 6 * 4 * 4 = 64 ft^3
For each cubic foot, it's safe to put 3 small fish, so the number of fishes that can be safely put into the tank is calculated with the following rule of three:
1 cubic foot -> 3 fishes
64 cubic feet -> X fishes
1/64 = 3/X
X = 64*3 = 192 fishes