请告诉我如何解答这个题目---**Question Number:** 5 **Question Stem:** 一块长方形菜地，一面靠墙，其他三面用篱笆围了起来（如下图），篱笆长为 40 米。这块菜地的面积是多少平方米？ **(Translation: A rectangular vegetable patch, one side against a wall, the other three sides are enclosed by a fence (as shown in the figure below), the fence length is 40 meters. What is the area of this vegetable patch in square meters?)** **Options:** (No options are provided in the image.) **Chart/Diagram Description:** * **Type:** Geometric diagram representing a rectangular area adjacent to a wall. * **Main Elements:** * A horizontal line at the top represents a wall (indicated by hatching above the line). * A rectangle is positioned below the wall, with its top side adjacent to the wall. * The interior of the rectangle is filled with small green symbols representing plants. * The three sides of the rectangle not against the wall are implied to be enclosed by a fence. These are the bottom side (parallel to the wall) and the two vertical sides (perpendicular to the wall). * A horizontal dimension line with arrows is placed below the bottom side of the rectangle. * A label "16 米" (16 meters) is placed below the dimension line, indicating the length of the bottom side of the rectangle. **Other Relevant Text:** * "如下图" (as shown in the figure below) * "篱笆长为 40 米" (fence length is 40 meters) * "面积是多少平方米" (What is the area in square meters?) * Unit: 米 (meter)