When the fence on one side of the vertical old wall is x meters long, the fence on one side of the wooden door is (X- 1) meters long, so the fence is now [20-(x- 1)-x] meters long, and because the old wall is 8 meters long, the remaining fence should not exceed 8 meters, that is [20-(x-65433]. =8, x of the solution < 0; =6.5, considering the practical problems, so 0 =