This study examined whether learning with heuristic worked examples can improve students' competency in solving reality-based tasks in mathematics (mathematical modeling competency). We randomly assigned 134 students in Grade 5 and 180 students in Grade 7 to one of three conditions: control condition (students worked on reality-based tasks), worked example condition (students studied worked examples representing a realistic process of problem-solving by fictitious students negotiating solutions to the tasks), and prompted worked example condition (students additionally received self-explanation prompts). In all three conditions, the students worked on the tasks individually and independently for 45 min. Dependent measures were mathematical modeling competency (number of adequate solution steps and strategies) and modeling-specific strategy knowledge. Results showed that although strategy knowledge could be improved through the intervention for fifth and seventh graders, modeling competency was improved only for seventh graders. The prompting of self-explanations had no additional effect for either fifth or seventh graders.