Adding 1/3 cup to 1/3 cup makes 2/3 cup. Adding fractions requires that the bottom numbers, or denominators, are the same. The top numbers, or numerators, are then added together.
In the above problem, the denominators are already the same. If they are not, a common denominator must be found. For instance, before adding 1/3 cup to 1/2 cup, it is necessary to find a denominator that is divisible by both two and three. The denominator in this case is six. To make 1/3 into a fraction with a denominator of six, both numbers are multiplied by two, and to make 1/2 into a fraction with a denominator of six, both numbers are multiplied by three. The problem becomes 2/6 cup plus 3/6 cup equals 5/6 cup.