In practice, mathematics education is blocked (i.e., teaching one topic at a time; CCSS, 2010), but research generally promotes interleaving (i.e., teaching multiple topics together; Rohrer & Taylor, 2007). For example, fraction arithmetic is blocked with students being taught fraction addition before fraction multiplication. Since students often confuse fraction operations to produce arithmetic errors, interleaved fraction arithmetic instruction might be more productive than blocked instruction to teach students to discriminate between the operations. Additionally, a cognitive task analysis suggests that fraction multiplication may be a prerequisite to fraction addition and thus reversing the blocking order may enhance learning. Two experiments with fraction addition and fraction multiplication were run. Experiments 1 and 2 show that interleaved instruction is generally better than the current blocked instruction. Experiment 2 provides evidence that blocking that reverses the standard order -- providing practice on fraction multiplication before fraction addition -- produces better learning.