def score_sentences(sen1, sen2):
    """
    Compares two sentences, find intersection and scores them
    :param sen1: (str) sentence
    :param sen2: (str) sentence
    :returns: score
    """

    s1 = set(sen1.lower().split())
    s2 = set(sen2.lower().split())
    score = 0
    if s1 and s2:
        avg = len(s1) + len(s2) / 2.0
        score = len(s1.intersection(s2)) / avg
    return score