beens.org Transistor Current Gain Challenge (doctest)

def transistor_gain(collector_current, base_current):
    """ (number, number) -> float

    Precondition: currents are both postive numbers

    Per instructions at http://goo.gl/VuUwy

    >>> transistor_gain(1,0.1)
    10.0
    >>> transistor_gain(.1,.3)
    0.3
    >>> transistor_gain(4,3)
    1.3
    >>> transistor_gain(10,10)
    1.0

    Write your code for this function below.
    """
    gain = ....

    return gain

if __name__ == '__main__':
    import doctest
    print (doctest.testmod())