def student_ranking(student_scores, student_names):
    """
    :param student_scores: list of scores in descending order.
    :param student_names: list of names in descending order by exam score.
    :return: list of strings in format ["<rank>. <student name>: <score>"].
    """
    student_ranking = []
    for index, score in enumerate(student_scores):
        result = f"{index + 1}. {student_names[index]}: {score}"
        student_ranking.append(result)

    return student_ranking