def letter_grades(highest):
    """Create a list of grade thresholds based on the provided highest grade.

    :param highest: int - value of highest exam score.
    :return: list - of 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
            56 <= "C" <= 70
            71 <= "B" <= 85
            86 <= "A" <= 100
    """

    increment = round((highest - 40) / 4)
    scores = []
    for score in range(41, highest, increment):
        scores.append(score)
    return scores