def future_value(interest, period, cash):
    """(float, int, int_or_float) => float
    Return the future value obtained from an amount of cash
    growing with a fix interest over a period of time.
    >>> future_value(0.5,1,1)
    1.5
    >>> future_value(0.1,10,100)
    259.37424601
    """
    if not 0 <= interest <= 1:
        raise ValueError('"interest" must be a float between 0 and 1')

    for d in range(period):
        cash += cash * interest
    return cash