The area to be fenced in is 20 ft by 45 ft, or 900 ft^2. But "25 per foot based on the area" is illogical. Please ensure that you have copied this problem down accurately, and be sure to provide units of measurement: did you mean $25 per square foot, or per foot, or what?
The fencing cost is usually dependent on the unit cost and the length of the yard perimeter. The perimeter is P = 2L + 2W, which here is P = 2(45 ft) + 2(20 ft), or 130 ft. If the fencing is 25 cents per foot, $130 ft of fencing would come to $32.50.
