6th grade math

Drusilla replaces the light bulb in the hall closet every 9 months and replaces the air filter every 6 months. She just replaced both items this month. After how many months will she replace both the light bulb and the air filter?

They will both be replaced the 18th month.

She will replace both at the same time in 18 months

Step-by-step explanation:

The first thing you have to do to find when she will change them at the same time, is to try with multiples of both, untill they match.

Hall Closet = every 9 months

Air Filter = every 6 months

1) 9-18-27-36-45

2) 6-12-18

We find that in the 18th month he will change both of them.