Zöe schedules advertising for a radio station. She must fill 12 minutes each hour with 30 second ads and 60 second ads. Zöe sold twice as many 60 second ads as she did 30 second ads. She defines x as the number of 30 second ads and y as the number of 60 second ads that she sold. Zöe writes the system 2 x = y and 30 x + 60 y = 12. Which statement identifies zöe’s error and the correction that is needed?.