Working with related denominators is preparation for establishing a general strategy for making common denominators when adding and subtracting fractions with unrelated denominators.
Students need time to develop an understanding of the processes involved in addition and subtraction of fractions, particularly for establishing the need for the denominators to be the same in order to complete the calculation.
Some students may have already developed general strategies for equivalence, but many will need representations such as grids and number lines to continue exploring the relationships between fractions with different denominators.
