Drusilla replaces light bulbs every 9 months and air filter every 3 months how long will it be until she changes both the light bulb and the air filter

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 monthsand (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 filteralthough she is changing air filters in between those times