Ok, that is big. The better to see the ratios with my dear... anyway, on to the math. Do you see how the height and radius of the tank form a triangle similar to any corresponding radius and height?
You can express the radius in terms of the height.
So now we can set up the related rate problem to determine the rate at a given height.
Standard volume equation:
Expressed in terms of height alone:
Since we want to know the rate, we need to differentiate with respect to time:
Now you are ready to begin substituting in known quantites -- the rate and the height. I will let you take over from here. Don't forget that since the water is falling the rate is negative.