Answer
4.8 hours.
Work Step by Step
When 2 things are working together to fulfill one goal, and one can do the goal in $x$ amount of time, and the other can do the goal in $y$ amount of time, the formula for how long it will take for both working at the same time is:
$${\frac{1}{\frac{1}{x}+\frac{1}{y}}}$$
Use this for the hose. One can fill the pool in 12 hours and the other can fill the pool in 8 hours. Plug in the formula and you will find how many hours it will take for both working together.
${\frac{1}{\frac{1}{8}+\frac{1}{12}}} = 4.8$
It will take 4.8 hours to fill the pool.
