As students explore the concept of adding fractions, they should begin to see that a common denominator will always be the product of the two denominators.
What to look for
If the denominators do not have a common factor, the rectangles in both unit squares are congruent. If the denominators have a common factor, however, the rectangles are not necessarily congruent, as displayed in the file.
If one part of a recipe calls for (2/3) cup of milk and another part of the same recipe calls for (3/4) cup of milk, how much milk do you need all together for the recipe?
Answer: (17/12) cups.