If Ann cleans in 12 hours then she cleans [tex]\frac{1}{12}[/tex] of the apartment per hour. Similarly Liz cleans [tex]\frac{1}{18}[/tex] of the appartment per hour. So together then can clean [tex]\frac{1}{12}+\frac{1}{18}=\frac{5}{36}[/tex] of the apartment for hour. Therefor it will take them [tex]\frac{36}{5}=7.2[/tex] hours to clean.
The general formula [tex]R_{c}=\frac{AB}{A+B}[/tex] can be used as a check if needed.
