def letter_grades(highest):
    """
    :param highest: integer of highest exam score.
    :return: list of integer lower threshold scores for each D-A letter grade interval.
             For example, where the highest score is 100, and failing is <= 40,
             The result would be [41, 56, 71, 86]:


             41 <= "D" <= 55 14
             56 <= "C" <= 70 14
             71 <= "B" <= 85 14
             86 <= "A" <= 100 14
    """
    thresholds = []
    intervals = []
    failing = 41
    # 100 - 41 = 59  59 / 4 = 15
    # 100 41 56 71 86   
    # 97 41 55 69 83 97-41 56/4 14
    # 85 41 52 63 74 85 85-41 44/4 11
    # 92 41 54 67 80 92-41 51/4 13
    # 81 41 51 61 71 81-41 40/4 10
    interval = 0
    interval = round((highest-failing) /4 ) 
    intervals.append(failing)
    intervals.append(failing + interval*1)
    intervals.append(failing + interval*2)
    intervals.append(failing + interval*3)
    return intervals