"Business Logic" is a generic term in the industry that usually refers to the rules that customers want a program to follow, based on their business model and how they need to do things. More generally, it can just mean the "core logic" of a program. So based on that, I would assume the business logic in a calculator would be the part that does the actual add/subtract/multiply/divide, separate from the GUI which simply collects data from the user, sends it to the business logic layer, and the displays the results. But I don't understand what is meant by "the class that does all of the work on the front end". The teacher seems to be referring to the GUI with that answer, not the core/business logic. It seems to me you'd be well justified in asking for more clarification from the teacher, not so much Google.