Answer
$800\ gal/hr$
Work Step by Step
Step 1. Assume $V\ (gal/hr)$ is amount of water emptied by a pipe per hour.
Step 2. Assume $d\ (in)$ is the diameter of the pipe.
Step 3. From the given conditions, we have $V=kd^2$ and $200=k(6)^2$, thus $k=\frac{50}{9}$
Step 4. For $d=12\ (in)$, we have $V=\frac{50}{9}(12)^2=800\ gal/hr$