def natural_bounds(rank, nb_ingredients):
    """
    Computes the upper and lower bounds of the proportion of an ingredient depending on its rank and the number of
    ingredients in the product given that they are in decreasing proportion order.

    Examples:
        >>> natural_bounds(2, 4)
        (0.0, 50.0)
        >>> natural_bounds(1, 5)
        (20.0, 100.0)

    Args:
        rank (int): Rank of the ingredient in the list
        nb_ingredients (int): Number of ingredients in the product

    Returns:
        tuple: Lower and upper bounds of the proportion of the ingredient
    """
    if rank == 1:
        return 100 / nb_ingredients, 100.0
    else:
        return .0, 100 / rank