You start by finding the total sq ft you need to cover so one board is 8x3 so that's 24 then multiply that by the number of boards which is 12 so you get 288 sq ft you need to cover. Then you divide by 125 which is how much one can could cover and you get 2.3 cans but you can't buy a third of a can do you round up to three. The three cans times the price of 3.50 gives you $10.50 as your final answer