light bulbs every 9 months : 9,18,27,36,45
air filters every 3 months : 3,6,9,12,15,18,21,24,27
she would change them both together (for the first time)at 9 months....and (for the second time) at 18 months...and again at 27 months...well, basically, every time she changes a light bulb, she is changing an air filter....although she is changing air filters in between those times
