Answer:
I have no clue what I could answer that question with other than (-6,-10)
If Ann cleans in 12 hours then she cleans

of the apartment per hour. Similarly Liz cleans

of the appartment per hour. So together then can clean

of the apartment for hour. Therefor it will take them

hours to clean.
The general formula

can be used as a check if needed.
I think the answer is 4/12 or 1/3