def _starsToPercentage (numStars):
    """
    Converts a given number of stars to its percentage

    :param numStars: teh number of stars as an integer or float
    :return: the percentage is the number is valid or None if it's not.
    """
    if numStars == 0:
        return 1
    elif numStars == 1:
        return 1.05
    elif numStars == 2 or numStars == 3:
        return 1.1
    elif numStars == 4 or numStars == 5:
        return 1.15
    elif numStars == 6 or numStars == 7:
        return 1.2
    else:
        return None