input: Write a python function to grade an exam based on a given answer key and student's answers.
  assert grade_exam(['A', 'B', 'D', 'C', 'D'], ['A', 'A', 'C', 'B', 'D']) == 20.0
  assert grade_exam(['A', 'B', 'C', 'D'], ['A', 'B', 'C', 'D']) == 100.0
  assert grade_exam(['A', 'A', 'A', 'A'], ['B', 'B', 'B', 'B']) == 0.0
output: def grade_exam(answer_key, student_answers):
      correct_count = sum(1 for key, answer in zip(answer_key, student_answers) if key == answer)
      return (correct_count / len(answer_key)) * 100