def calculate_lives_touched(cov_pop_burden_df):
    """Creates a column for lives touched by multiplying relevant columns
       Inputs:
           cov_pop_burden_df - a df that must contain columns 'target_pop', 
               'coverage' and 'prob_cover'
       Returns:
           a df with a lives_touched column
    """
    cov_pop_burden_df['lives_touched'] = (cov_pop_burden_df['target_pop']*
                                          cov_pop_burden_df['coverage']*
                                          cov_pop_burden_df['prob_cover'])
    return cov_pop_burden_df