def letter_grades(highest):
    """
    :param highest: integer of highest exam score.
    :return: list of integer score thresholds for each F-A letter grades.
    """
    interval_size = (highest - 40 ) // 4
    intervals = [41]
    for i in range(1,4):
        intervals.append(intervals[0] +  i * interval_size)
    return intervals