## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$$\frac{36}{7}{hours}$$
We add the rates to find their combined rate: $$\frac{1}{9}+\frac{1}{12}\\ \frac{4}{36}+\frac{3}{36}\\ \frac{7}{36}$$ Thus, taking the reciprocal, their combined time is: $$\frac{36}{7}{hours}$$