I will give you the steps but try to do the problem by yourself.
Start by finding the volume of the tank
Multiply 1.75 ft and 1.25 ft then covert the sum to inches by multiplying it by 12.
Then multiply that amount by the ll inches and you have the volume.
Since the tank is 20% full divide the volume by five, take the number you get from that and subtract it from the origanl volume.
Now you have the amount you need to fill.
Since it is 3$ a gallon and 231 cubed inches is a gallon 77 cubed inches is 1$.
Divide the amount you need to fill by 77 and you get the cost to fill the tank.