When Millie decides to furtively glance at the answer provided in the back of the text to get an idea of how the solution should look like, she is using the problem-solving heuristic technique of "rule of thumb".
This technique is a mental short-cut that humans use to help with the process of problem-solving. When using the "rule of thumb" technique, Millie is applying a broad approach to problem solving. Her looking at the answer before starting to work on the exercise, is giving her an approximation of the solution without conducting much an in deep research.
