def mixing(c1, c2, f):
    """
    mixing creates a mixing model between two endmembers

    Inputs:
        c1 = concentration of endmember 1
        c2 = concentration of endmember 2
        f = fraction of endmember 1 in the model

    Returns:
        cm = concnetration of the mixture
    """
    cm = c1 * f + c2 * (1 - f)
    return cm