def frequency(observed_mutant_count, number_of_cells):
    """ Calculate mutation fraction (or frequency) per culture.

    Frequency calculated as r/N.
    where r is observed number of mutants
          N is a number of cells in a culture

    :param observed_mutant_count: observed number of mutants in a culture
    :param number_of_cells: number of cells in a culture
    :return: mutation fraction per culture
    """
    return observed_mutant_count / number_of_cells