Good programs are built from small functions that each do one job, combined by others:

def average(numbers):
    return sum(numbers) / len(numbers)

def grade(mark):
    if mark >= 70:
        return "A"
    if mark >= 50:
        return "C"
    return "U"

def report(name, marks):
    avg = average(marks)
    print(f"{name}: average {avg:.1f}, grade {grade(avg)}")

report("Sam", [72, 81, 66])
report("Alex", [45, 52, 38])

Each piece can be read, tested and fixed on its own. Breaking a problem into pieces like this is called decomposition, and it is the core skill of programming. A useful habit: if you can't describe what a function does in one short sentence, it probably wants splitting.