def subtract_9_if_over_9(lst):
    """
    Takes in a list of numbers and returns a list where
    all values over 9 are subtracted by 9.

    >>> subtract_9_if_over_9([10, 8, 18, 9, 16, 6, 16, 5, 14, 3, 14, 6, 10, 5, 8])
    [1, 8, 9, 9, 7, 6, 7, 5, 5, 3, 5, 6, 1, 5, 8]
    """
    for i in range(len(lst)):
        if lst[i] > 9:
            lst[i] -= 9
    return lst