This topic focuses on subtracting fractions by finding the lowest common denominator. It involves dealing with mixed fractions and requires changing the denominators of two fractions to perform the subtraction. Specifically, the problems present pairs of mixed fractions and task the learner with determining the ideal common denominator for subtraction. This is an advanced exercise on fraction addition and subtraction, testing the user's understanding of denominators and simplification within the context of subtracting mixed numbers.