Solve word problems involving addition and subtraction of fractions referring to the same whole, including cases of unlike denominators, e.g., by using visual fraction models or equations to represent the problem. Use benchmark fractions and number sense of fractions to estimate mentally and assess the reasonableness of answers. For example, recognize an incorrect result 2/5 + 1/2 = 3/7, by observing that 3/7 < 1/2.
Solve word problems involving the addition and subtraction of fractions with unlike denominators. Recognize mistakes in adding unlike denominators. For example, 2/5 + 1/2 = 3/7 because you can't simply add numerators and denominators.