Rachael paints a room in 9 hours, so her work rate is 1/9 of work per hour.
Kelsey paints the same room in 15 hours, so her work rate is 1/15 of work per hour.
Together, their combined work rate is
[tex]\dfrac{1}{9}+\dfrac{1}{15}=\dfrac{8}{45}[/tex]
Which means it will take them 45/8 of a hour to complete the job together.
