You need to be able to produce a report card for all students. Before you can do that, you will need the capability to calculate the GPA (grade point average) for a given student. A student receives a number of grades. You store a student's grade by sending the message addGrade to a Student object, passing along the letter grade. When asked for the GPA, the student uses its stored grades to calculate a GPA.
You will supply grades as decimal numbers, known in Java as floating-point numbers.