First, determine the maximum water capacity of the tank, to do so, you have to calculate its volume using the formula:
The tank has a length of l=20in, a width of w=10in, and a height of h=12in, you can calculate the volume as follows:
Second, the tank is filled up to 4/5 of its capacity, to determine how much water 4/5 represent, you have to multiply the volume of the tank by the said fraction:
When the tank is filled at 4/5 of its capacity, the volume of water is 1920in³