input: Write a function to assign letter grades based on scores.
  assert assignGrades([95, 82, 75, 66, 50]) == ["A", "B", "C", "D", "F"]
  assert assignGrades([100, 89, 76, 65, 55]) == ["A", "B", "C", "D", "F"]
  assert assignGrades([91, 85, 72, 64, 40]) == ["A", "B", "C", "F", "F"]
output: def assignGrades(scores):
      grades = []
      for score in scores:
          if score >= 90:
              grades.append("A")
          elif score >= 80:
              grades.append("B")
          elif score >= 70:
              grades.append("C")
          elif score >= 60:
              grades.append("D")
          else:
              grades.append("F")
      return grades