Question

Ann cleans an apartment in 12 hours and liz cleans an apportment in 18 hours. How long would it take to clean an appartment together?

Answer 1

If Ann cleans in 12 hours then she cleans
`(1)/(12)` of the apartment per hour. Similarly Liz cleans
`(1)/(18)` of the appartment per hour. So together then can clean
`(1)/(12)+(1)/(18)=(5)/(36)` of the apartment for hour. Therefor it will take them
`(36)/(5)=7.2` hours to clean.

The general formula
`R_(c)=(AB)/(A+B)` can be used as a check if needed.