we have a garden that measures 17 by 20 feet. we want to pour a concrete for a 3-foot-wide sidewalk side walk around the garden. to make the forms for the concrete, we will need to buy some 2 by 4 inch lumber.how many feet of lumber will we need just for the perimeter of the walk? (consider both the inside and outside perimeter). Johnson, Ken."Crossing the River With Dogs" chapter 3 page 65.