This math topic focuses on practicing fractional subtraction by determining the lowest common denominator (LCD). The problems involve the subtraction of two fractions where the denominators must be changed to facilitate the operation. It is designed to develop and test the ability to find what LCD is most suitable for subtracting pairs of fractions with different denominators, reflecting a basic level of difficulty. This is part of a more advanced unit on mixed fraction addition and subtraction. The problems include multiple-choice answers where students need to choose the correct LCD for subtraction.