3 3/4 inches = (3 3/4 / 12) feet = 0.3125 feet
120 feet requires ------- x number of board.
120 feet/0.3125 feet / board = 384 boards.
1 board costs 1.40
384 boards cost x
1.4 * 384.00= x*1 = x
537.60 dollars.
Note if you need to do this by dimensional analysis, it will look like this.
120 feet/fence [ 1 board / 3.75 inches] [ 12 inches/foot] [1.40 dollars / board]
All units that can be cancelled should be.
The result is 537.6 dollars / fence which is the answer you got.
