How can the addition and subtraction of fractions be generalized?

Students add and subtract fractions with denominators within 100.

Addition and subtraction of fractions is facilitated by representing the fractions with common denominators.

Denominators are related if one is a multiple of the other.

Multiplication of one denominator by the factor that relates it to another denominator achieves common denominators.

The product of the denominators of two fractions provides a common denominator.

Fractions with common denominators have the same units.

Any numbers with the same unit can be compared, added, or subtracted.