Standards for School Mathematics, page 35

The development of rational number concepts is a major goal for grades 3-5, which should lead to informal methods for calculating with fractions. For example, a problem such as 1/4 + 1/2 should be solved mentally with ease because students can picture 1/2 and 1/4 or can use decomposition strategies, such as 1/4 + 1/2 = 1/4 + (1/4 + 1/4).