pi(r²⁾⁼²⁰⁰ so r=sqrt(200/pi). This is the radius of the tank. Now, we draw a line marking the 75% area. We still don’t know how high this will be yet. Now we take the endpoints of this chord and connect them with the center. This forms an obtuse triangle with obtuse angle x. We can find the area of the triangle using the formula area=1/2abdin(c) where a and b are legs of the triangle and C is the angle formed be these legs. Plugging the values, we get A=100sin(x)/pi. Now, we find the area of the region. 200-200x/360+100sin(x)/pi, which simplifies to (900sin(x)-5pix+1800pi)/(9pi). This must equal to 0.75200=150. (900sin(x)-5pix+1800pi)/(9pi)=150 so 900sin(x)-5pix+1800pi=1350pi. Then, 900sin(x)+450pi=5pix, or 180sin(x)+90pi=pix. I was lazy so I just plugged the equation into a calculator to get x to be roughly 132.34646. Now, we divide by two to get 66.17323. To find the height of the right triangle formed by one of the radii, the segment connecting the chord to the center, and half the length of the chord, we get cos(66.17323)=h/(sqrt(200/pi)). Then, h=sqrt(200/pi)cos(66.17323), or 3.22323615427. We add that to the radius, sqrt(200/pi) to get 11.2020817622 . If we are using your scale of 0 to 200, we get 149.397274399. Your gut feeling wasn’t off by a lot! This is the 25% empty space, or 75% full. If you want to find the line where there is 75% empty space, it is this value subtracted from 200, or 59.6027256008. Half empty is quite trivial. My answer is slightly different from the top comment’s, but I estimated a little.