This math topic focuses on calculating the length of a part side of a rectangle based on the given areas of the entire rectangle and a specific part of it. It is intended to develop understanding of area concepts and proportional reasoning in the context of rectangles. The problems are set at an introductory level within a broader unit on area and perimeter logic. Each question presents a different scenario with a full rectangle's area and a partial area, asking for the length of the part side, offering multiple choice answers.