Okay. fist off don't think of them as 5 units each, but 1. so if you were to put it on graphing paper and box squares each time. you would be able to get 8 in for both sides to make a square. Area then would be 1600ft squared because 8 ×5 is 40 and 40×40 is 1600. then you calculate how many squares you used. 8×8=64. and 80-64=16 so 16 pieces are left over. then 9×9 (increasing the square) is 81. so 17 pieces to increase it.
